http://www.perlmonks.org?node_id=1020708

SimplySimple has asked for the wisdom of the Perl Monks concerning the following question:

I'm a beginner to Perl, and I have a question about assigning values to the variable $_. I'm trying to get "Test A" as the output and I get no output instead. And here is the slice of code I want to fix.

"Test A"or"Test B";
print;

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May someone help me to modify the code to get the expected result? Thanks a lot.

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Re: Implicit assignment to $_
by blue_cowdawg (Monsignor) on Feb 26, 2013 at 16:10 UTC

    This is a prime example of why I preach so heavily about use strict;. If you had invoked that pragma you'd have seen the warning: 

    Use of uninitialized value $_ in print at sopw-simple.pl line 6.

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    Quoting from Gabor Szabo on his website http://perl5maven.com/the-default-variable-of-perl: 
    In Perl, several functions and operators use this variable as a default, in case no parameter is explicitly used. In general, I'd say you should NOT see $_ in real code. I think the whole point of $_ is that you don't have to write it explicitly.

Well, except when you do.

Having a default variable is a very powerful idea, but using it incorrectly can reduce the readability of your code.
    Just for grins I tried: 
    $_ = "TestA" or "TestB";

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    and Perl was not happy with that issuing: 
    Found = in conditional, should be == at ./sopw-simple.pl line 4.
Useless use of a constant (TestB) in void context at ./sopw-simple.pl 
+line 4.
TestA

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    My suggestion is read the web page I provided above for some hints on what you can and can't do with $_. Just a thought...

    Peter L. Berghold -- Unix Professional
    Peter -at- Berghold -dot- Net; AOL IM redcowdawg Yahoo IM: blue_cowdawg
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      "This is a prime example of why I preach so heavily about use strict;. If you had invoked that pragma you'd have seen the warning:

      "Use of uninitialized value $_ in print at sopw-simple.pl line 6."

      Nuh-uh... strict does no such thing.

      strict does three things:

      • Throws compile-time errors for undeclared variables.
      • Throws compile-time errors when barewords are seen which don't correspond to subs, and don't look like the package name in a class method call.
      • Throws run-time errors when symbolic references are used, or more generally when trying to dereference a variable which does not contain a reference.

      strict never issues any warnings. And strict doesn't care about variables being undef (unless you try to dereference them).

      package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name
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            Nuh-uh... strict does no such thing.

        Really? That's a copy/paste from my terminal. Wish there was a way to post screenshots to PM.

        Peter L. Berghold -- Unix Professional
        Peter -at- Berghold -dot- Net; AOL IM redcowdawg Yahoo IM: blue_cowdawg
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      Despite all correct (!!) advices given before:

      print for ("Test A" or "Test B");

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      Best regards
      McA

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Re: Implicit assignment to $_
by grondilu (Friar) on Feb 26, 2013 at 16:25 UTC

    Your code: 

    "Test A"or"Test B";
print;

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    suggests you think $_ refers to the last evaluated expression. It does not. $_ is not python's underscore in interactive mode.

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Re: Implicit assignment to $_
by arnaud99 (Beadle) on Feb 26, 2013 at 16:08 UTC

    Hi, I am unsure about what you are trying to achieve here as there is very little of the code.

    $_ is a global variable and quite a few places in perl use $_ as the default.

    To know more use 'perldoc perlvar' and you will have a good explanation of where and how $_ can be used.

    Below is a small program that uses $_ as the default for both the while loop and the print statement.

    I hope this helps.

    Arnaud. 

    use strict;
use warnings;
use autodie;

while(<DATA>) {
    print;
}

__DATA__
Test A
Test B

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Re: Implicit assignment to $_
by tobyink (Canon) on Feb 26, 2013 at 16:59 UTC 
    $_ = ("Test A" or "Test B");
print;

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    package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name
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Re: Implicit assignment to $_
by pvaldes (Chaplain) on Feb 26, 2013 at 17:05 UTC

    I'm trying to get "Test A"

    print "Test A" or die "Test B is not acceptable, its purpose here is merely esoteric";

    (Yes, you are doing wrong) Is "Test A" a six character's string, a filehandle, a variable, a macguffin?

    print  $test_A;     # looking for this?

    We simply need some more clues

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