Simple perl environment variable substitution

vsub.pl

#!/usr/bin/perl

use strict;
use warnings;

my $num_args = $#ARGV + 1;
if ($num_args != 2) {
    print "\nUsage: vsub.pl [input file] [output file]\n";
    exit 1;
}

# Slurp whole file into content variable
my $file_name = $ARGV[0];
my $output_file = $ARGV[1];
open my $fh, $file_name or die "Could not open $file_name: $!";
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;

# Replace all keys in the environment with the value
foreach my $key (keys %ENV) {
  $key = quotemeta $key; # escape regex metachars if present
  $content =~ s/#{$key}/$ENV{$key}/g;
}

open (OUTFILE, ">>$output_file");
print OUTFILE $content;
close (OUTFILE);