CHUNK:
while ( ( my $pos = pos $sentence ) < length $sentence ) { 
    for my $phrase ( @phrases ) {
        if ( my $index = index($sentence, $phrase ) >= $pos ) {
            my $length = length $phrase;
            substr($sentence, $index + $length, 0, '#');
            substr($sentence, $index, 0, '#');
            pos $sentence = $index + $length + 2;
            next CHUNK;
        }
    }
    last CHUNK;
}

##</code><code>##

my @split = split /($phrase1|$phrase2)/, $sentence;
@phrases = @split[map { 2 * $_ + 1 } 0 .. ($#split - 1)/2];