#!/usr/bin/perl -w
use strict;

sub prefix	{
	my ($line, $look_for, $prefix, $flip) = @_;
	return $line unless $look_for && $line =~ /$look_for/;
	my @arr = split /(<[^>]*>)/, $line;
	for my $cnt (0..$#arr)	{
		if ($arr[$cnt] =~ /$look_for/)	{
			$arr[$cnt-2] .= $prefix if $cnt > 1 && !$flip;
			$arr[$cnt+2] = $prefix . ' ' . $arr[$cnt+2] if $cnt < $#arr-1 && $flip;
		}
	}
	return join '', @arr;
}


my $line = "<5b>I <5c>like <5d>tacos\n";
print prefix ($line, 'tacos', 'yummy');
print prefix ($line, 'like', 'yummy', 1);

__OUTPUT__
<5b>I <5c>like yummy<5d>tacos
<5b>I <5c>like <5d>yummy tacos