It was a great revelation when I first learned of the 'e' flag for regexes (when I learned of the double 'ee' it was more fear inspiring than revelatory). This is a simple play at a kilometer/mile converter. I'd love to see others' samples here of regexes that execute code.
my $sentence = ( join " ", @ARGV ) ||
    "Give me a sentence containing kilometers or miles or both.\n" .
    qq|   Eg (note quotes): "I live 6 miles from here."|;

$sentence =~ s!                            # we're substituting
        \b(\d+(?:\.\d+)?)\b                 # find a number
        \s+                                 # followed by space
        (kilometer|mile)s?                  # "km"s or "m"s
        !                                   # 1/2 done
        sprintf "%.1f %s%s",                # convert,
        $1 * ($2eq'mile'?1.6:.63),          # if "m"s, use 1.6
        ($2eq'mile'?'kilometer':'mile'),    # if "km"s, use .63
        ($1*($2eq'mile'?1.6:.63))==1?'':'s' # do the math
        !xieg;                              # ignore, execute, repeat

print $sentence, "\n";

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In reply to mile/kilometer converter (category: stupid regex tricks) by Your Mother

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