my %transformation_by_filetype => (
  "c" => {
    "foo" => qq{
You had a foo.
},
    "foo" => qq{
Bar is great.
},
  },
);
Then run something like this:
<code>
#! /usr/bin/perl -w
use strict;

for my $file (@ARGV) {
  my $filetype = determine_filetype($file);
  my $transformation = $transformation_by_filetype{$filetype};
  rename($file, "tmp.$$") or die "Can't rename $file to tmp.$$: $!";
  open (IN, "<", "tmp.$$") or die "Can't read tmp.$$: $!";
  open(OUT, ">", $file) or die "Can't write $file: $!";
  while (my $line = <IN>) {
    print OUT $line;
    for my $pattern (keys %$transformation) {
      if ($line =~ /$pattern/) {
        print OUT $transformation->{$pattern};
      }
    }
  }
}

sub determine_filetype {
  my $filename = shift;
  $filename =~ s/.*\.//;
  return $filename;
}