#!/usr/bin/perl -w

use strict;
use warnings;

my  $var_one;
    $var_one = 'now defined';

sub copy_file {
  while( my $x = <DATA>) {
      $x =~ s|(\$\w+)|$1|eeg; 
      print $x;
  };
}

copy_file();

__DATA__
The variable is $var_one.