I think what you want is simply:

    use strict;
    use warnings;
    use File::Basename;

    $org_fn = $ARGV[1];
    my $base = basename $org_fn;
    print "Basename is $base\n";
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Note that File::Basename is a great way to get the name of the script itself, too.

I usually do this:

    my $iam = basename $0;
    my $syntax = "
        syntax:  $iam ...
    ";
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and then don't have to worry about changing the program name; the syntax message stays correct.