You could set it to a blank string if not defined like this

#!/usr/bin/perl

use warnings;
use strict;

$ARGV[0] ||= '';

if ($ARGV[0] eq 'TEST') {
    print "$ARGV[0]\n";
}
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or check for it being defined like this

#!/usr/bin/perl

use warnings;
use strict;

if (defined($ARGV[0]) && $ARGV[0] eq 'TEST') {
    print "$ARGV[0]\n";
}
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I prefer the second option.