use strict;
use warnings;

my $input = '-L/usr/local/lib -lmylib -lz';
$input =~ s!^-L(.+)\s+\K-l(.+)(?=\s+.+)!$1/$2.a!;

print $input;

##</code><code>##

-L/usr/local/lib /usr/local/lib/mylib.a -lz