use strict;
my $str = '(S)FSK-69/23/GFK/ 1 2
(S)FSK-69/23/GFK/12
FS-4444g/23g/F/1g1
FS-4444/23g/F/11';
my $regex = qr/(?<=^FS-[a-zA-Z0-9]*)[a-z]|[\p{Zs}\ta-z](?=[a-zA-Z0-9\p{Zs}\t]*$)/mp;
my $subst = '';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html