#!/usr/bin/perl
use Data::Dumper;

while (<DATA>) {
  $in{$1} = 1 if ($_ =~ (/^\s*(\$\w+.*)/));
}
@a = keys %in;
print Dumper \@a;

__DATA__
$first = 1;
$first = 1;
  $second = 2; #some comment
#  $third = 3;