#!/usr/bin/perl -w

use strict;

my %subs =
(

  qr/(\d+)F/i => sub { 
    warn "Matched F with $1\n";
    return sprintf("%.u" , ($1 - 32) / 1.8) . 'C'
  },
  qr/(\d+)C/i => sub {
    warn "Matched C with $1\n";
    return sprintf("%.u" , ($1 * 1.8) + 32) . 'F'
  },

);

my $html = do { local $/; <DATA> };

warn $html;

foreach my $key (keys %subs)
{
  warn "checking key: $key\n";
  $html =~ s/$key/$subs{$key}->()/eg;

  warn $html;
}

warn $html;

__DATA__
convert 180F to C
convert 180C to F
convert 30" to cm
etc


##</code><code>##

convert 180F to C
convert 180C to F
convert 30" to cm
etc

checking key: (?i-xsm:(\d+)C)
Matched C with 180
convert 180F to C
convert 356F to F
convert 30" to cm
etc

checking key: (?i-xsm:(\d+)F)
Matched F with 180
Matched F with 356
convert 82C to C
convert 180C to F
convert 30" to cm
etc

convert 82C to C
convert 180C to F
convert 30" to cm
etc