my ($str) = $input =~ m#^-L(((?!\s*-).)*)#;
$input =~ s#-lmylib#$str/mylib.a#;

##</code><code>##

#
# Input         : -L/usr/local/lib -lmylib -lz
# Desired output: -L/usr/local/lib /usr/local/lib/mylib.a -lz
#
use strict;
use warnings;

# The input string
my $input      = '-L/usr/local/lib -lmylib -lz';

# The meat
my ($str) = $input =~ m#^-L(((?!\s*-).)*)#;
$input =~ s#-lmylib#$str/mylib.a#;