Something simple like this could be a start (with filenames given on the command line):

use strict;
use warnings;
my %identifiers;
undef $/;
undef @identifiers{ /((?:sub\s+|[\$@%])[a-zA-Z_][a-zA-Z_0-9]*)/g } whi
+le <>;
print join "\n", keys %identifiers;
[download]

but there are more things to find, like module names etc.