#!/usr/bin/perl -wl

    use strict;

    $_ = "3 L-homoserine";

    /(\w-\w)/;
    print $1;

    s/(\d*\s*)L-homoserine/${1}2-Amino-4-hydroxybutyric acid/;
    print;

    $_ = "L-homoserine";

    s/(\d*\s*)L-homoserine/${1}2-Amino-4-hydroxybutyric acid/;
    print;

    __END__
    Prints:

    L-h
    3 2-Amino-4-hydroxybutyric acid
    2-Amino-4-hydroxybutyric acid