If you run your code with perl -Dr (assuming your perl interpreter is compiled with debugging enabled), you'll see what I can see now too:

the regexp engine works and works and works...

However, I cannot see yet exactly what the solution is; at first sight, the regexp seems to be only extremely inefficient via the backtracks when it does _not_ find what it looks for.

Update.

A work-around to avoid the heavy backtracking when the wanted terms are not to be found in the wanted order might look like

    if ($content =~ /$par2.*$par1/i) {
        if ($content =~ /\b($par2)(\W+(?:\w*\W*){1,$distance})?($par1)
+\b/i){
            warn "IF 2";
            my ($par1, $par2, $par3) = ($1, $2, $3);
            $content =~ s/$par1\Q$par2\E$par3/<$tag$class> $par1<\/$ta
+g>$par2<$tag$class> $par3<\/$tag>/gi;
        }
    }
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That works for me, but I guess there should be some nicer solutions too.

Update2 Looks like using a regexp like

    if ($content =~ /\b($par1)(\W+(\w+\W+){0,$distance})($par2)\b/i) {
[download]

works OK and also avoids the excessive backtracking for unsuccessful lookups. You'll have however to add an $4 and use it instead of $3 for the extra new match introduced with this.