Hello fkento, and welcome to the Monastery!

From Truth and Falsehood:

The number 0, the strings '0' and "", the empty list (), and undef are all false in a boolean context. All other values are true.

So, since the string “false” is none of 0, '0', the empty string, the empty list, or undef, it is true in Perl.

Update: Sorry, misread the question. The answer lies in the distinction between the operators == and eq. The former is for numerical comparison, the latter for string equality. See Equality Operators.

12:43 >perl -E "say (('false' == 'true') ? 'same' : 'different');"
same

12:43 >perl -E "say (('false' eq 'true') ? 'same' : 'different');"
different

12:43 >
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Hope that helps,

Thanks Athanasius. But I'm just trying to compare two strings. Here is a simpler case:

  if ( 'f' == 't' ) {
    print "f equals t\n";
  } else {
    print "f does not equal t\n";
  }
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Output:

f equals t
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How does one compare strings?

> How does one compare strings?

eq

Cheers Rolf

Try this:

if ( 'f' eq 't' ) {
    print "f equals t\n";
  } else {
    print "f does not equal t\n";
  }
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Compare '15' eq '15.0' versus '15' == '15.0'! Or if this looks artificial, let's assume you've read one of the numbers from a XML file and the other from STDIN. Now both are stored as strings by Perl, strings that will be converted to numbers if needed, but still what you read from both sources were strings! How should Perl guess whether you wanted to compare them as strings or as numbers? And if Perl did try to guess, how would you make sure it guesses right?

A single comparison operator is fine for strongly typed languages, but for weakly typed like Perl and JavaScript a pair of operators works much better. Actually the shared operator for string and number comparison and another for addition and concatenation is what I dislike most about JavaScript and what leads to huge loads of errors in JavaScript code in general. formFieldName.value + 1 anyone?

Jenda
Enoch was right!
Enjoy the last years of Rome.

fkento:

Nearly all values are true. False is reserved for a select few values like 0, '0' and ''.

Roboticus@Roboticus-PC ~
$ perl t.pl
0 is false
0 is false
foo is true
bar is true
fkento is true
true is true
false is true
yes is true
no is true
 is false

Roboticus@Roboticus-PC ~
$ cat t.pl
#!/usr/bin/perl
use strict;
use warnings;

for my $t (0, qw(0 foo bar fkento true false yes no), '') {
  print "$t is ", ($t ? 'true' : 'false'), "\n";
}
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Remember--text strings are just strings, even if they say "false" or "true". You should read the perldata (third paragraph after 'scalar' heading) for the basics.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

What am I missing?

Tip #1 from the Basic debugging checklist: warnings

Argument "true" isn't numeric in numeric eq (==) at  line 11 (#1)
    (W numeric) The indicated string was fed as an argument to an oper
+ator
    that expected a numeric value instead.  If you're fortunate the me
+ssage
    will identify which operator was so unfortunate.
    
Argument "false" isn't numeric in numeric eq (==) at  line 11 (#1)
false=true
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When I execute:

#!/usr/bin/perl
use strict;
use warnings;

my $str = 'false';

if ($str == 'true') {
    print "$str=true\n";
} else {
    print "$str=false\n";
}
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I get:

Argument "true" isn't numeric in numeric eq (==) at noname.pl line 7.
false=true
Argument "false" isn't numeric in numeric eq (==) at noname.pl line 7.
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which is a pretty good heads up! Always use strictures (use strict; use warnings; - see The strictures, according to Seuss).