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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