All you have to do is put the following at the top of your script:

use warnings;
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Add the following as well to report on all problems with your code:

use strict;
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Warning example for undefined variable:

use warnings;
use strict;

my $x;
print "$x\n";
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Output:

Use of uninitialized value $x in concatenation (.) or string at script
+.pl line 4.
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strict error for undeclared variable:

use warnings;
use strict;

print "$y\n";
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Output:

Global symbol "$y" requires explicit package name (did you forget to d
+eclare "my $y"?) at script.pl line 4.
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If it's for debugging, another way to tell the difference between the empty string and an undef value is to use Data::Dumper or Data::Dump. The first one requires no installation, but one of the reasons I like the second one better is because it displays non-printable character explicitly with escape sequences by default (it's the Useqq option in Data::Dumper), so it helps tell the difference between undef, "", "\n" or even "\0". As far as I'm concerned, whenever it's for debugging I don't print it I dump it.

Edit: eg:

use Data::Dump qw( pp );
pp undef;
pp "";
pp "\n";
pp "\0";
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NB: of course those two modules are also useful for displaying complex data structures.

more ways are possible. You might profit the ternary operator described in perlop too: IF CONDITION ? THEN : ELSE

use strict;
use warnings;

my $scalar; # starts empty ie undef

                                                   # IF        THEN   
+   ELSE
print "\$scalar now holds the following value: [", $scalar ? $scalar :
+ 'UNDEF',"]\n";


# oops more correct as afoken pointed below:

print "\$scalar now holds the following value: [", defined $scalar ? $
+scalar : 'UNDEF',"]\n";

# update:
# infact, unless you check with 'defined' a zero can mislead you:
$scalar = 0;
print "\$scalar now holds the following value: [",  $scalar ? $scalar 
+: 'UNDEF',"]\n";
print "\$scalar now holds the following value: [", defined $scalar ? $
+scalar : 'UNDEF',"]\n";
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L*

print "\$scalar now holds the following value: [", $scalar ? $scalar : 'UNDEF',"]\n";

It seems a defined has managed to escape from your posting. ;-)

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

Hello Nicpetbio23!,

Just to add something here to analytical explanation by fellow monk stevieb. You can use also the functions undef and defined.

Usually you do not check for a scalar as my example bellow but if a scalar exists specifically in a hash (usually). But just for demonstration purposes see bellow:

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

my $scalar = undef;

if (defined $scalar) {
    print "\$scalar is defined\n";
}
else {
    print "\$scalar is undefined\n";
}

__END__
$ perl test.pl
$scalar is undefined
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Hope this helps, BR.