When comparing a value against a constant, always write the constant to the left side like this:

if (undef = $WCkey) { ...
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If you make the error you made in your original post, Perl will immediately tell you.

Also, the function for checking whether a value is undef or not is the defined function.

Update: The if (undef = $WCkey) { ... is not a typo but intended for demonstration. Perl raises a runtime error when you use

if (undef = $WCkey) { ...
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instead of

if (undef == $WCkey) { ...
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while it doesn't even raise a warning if you write the "comparison" the other way around:

>perl -we "if (undef = $foo) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.
Modification of a read-only value attempted at -e line 1.

>perl -we "if ($foo = undef) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.
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So, in the spirit of building a defensive habit, write the constant to the left side of comparisons for equality.

Sorry about my original post, Now the problem is, I know defined and how to use it but the problem here is, if those variable are "undef" do some thing else, if (defined($key)) { .... } but is there any thing like this if (undefined($key)) { .... } Any suggestion???

You have not programmed much, have you? The perlop manpage does not hide how to invert logic expressions. See logical not for how to execute something if a condition is not true.

For futher reading, I recommend looking at Truth Tables and potentially then Boolean Logic.

In perl-5.10 and later you can also use the smart-match operator:

if ($WCkey ~~ undef && $SCeky ~~ undef && ... ) {  }
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why dont you try something like below

if (! defined $WCkey)

cheers srikrishnan
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If I try that I get

Semicolon seems to be missing at 2 line 4.
syntax error at 2 line 5, next token ???
Execution of 2 aborted due to compilation errors.
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Just kidding :D code tags intended for code/data/error messages

If you have to do these checks again and again, you can think of creating a dedicated sub routine for that.

sub all_defined {
    for my $i ( @_ ) {
        #return 0 if !defined $i;
        return if !defined $i;
    }
    return 1;
}

if ( all_defined( $WCkey, $SCkey, $Pkey, $Wkey, $Ukey ) ) {
    print "bla bla";
}
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return 0 has a potentially dangerous effect. It will evaluate as true if your subroutine is called in list context. Just a bare return is usually a better idea.

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