Place the following in a module called WarnUsedOnce.pm.

    package WarnUsedOnce;

    use Filter::Simple sub {
        my $self = shift;

        s/my//g; # Too simple
    };

    1;
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Then use the package in the code you want to test, and run the program with perl -c program.pl:

    use WarnUsedOnce;

    my $var1 = 1;
    my $var2 = 1;
    my @array;

    $var2 = 1; # $var2 used twice

    # Turn the warnings off for the following code
    no WarnUsedOnce;

    my $var3 = 2;

    __END__
    Warnings:

    Name "main::array" used only once: possible typo at test.pl line 5
+.
    Name "main::var1" used only once: possible typo at test.pl line 3.
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Some caveats:

• The my substitution is far too simple, it is only an example..
• You will also get a lot of "Useless use of a variable in void context" warnings with -w.
• Unused variables with the same name but in different scopes won't be detected.
• It probably won't work for pathological cases which in Perl means simple everyday cases. ;-)

So treat this as a proof of concept only.

--
John.

Abigail

#!/usr/bin/perl -w 
use strict;
my %seen;
while (<>) { s![\$\%\@]([a-zA-Z]\w*)!$seen{$1}++!eg  }
my @once = sort grep { $seen{$_} == 1 } keys %seen;
if (@once) {
    print "Possible unused variables found: \n\n", join("\n", @once), 
+"\n";
}
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Now, that's hardly foolproof. In any reasonably complicated program it's likely to show a lot of false positives. It might also miss some cases where a variable name is used in more than one type - $foo and @foo for example. A more complicated regex could be devised to deal with that case. Worse, if a variable is declared in multiple scopes but never used then it won't be in the list.

But for your test case it works! If I put my code in use.pl and yours in foo.pl:

$ perl use.pl foo.pl
Possible unused variables found: 

var_a
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-sam

It would fail on a simple piece of code like this:

    {my $var;}
    {my $var;}
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Both variables are unused. "Unused" would mean only declared in a certain scope, not only declared in the entire program.

Abigail

Writing a regex that parses Perl scopes is left as an exercise for the reader. Ultimately that's what tools like lint have done for C - they actually parse the C code just like a compiler would and do pattern matching to find bad code.

-sam