use strict;
use warnings;

my @flist = qw/MY.switch.Org.1st.txt router.2nd.Org.MY.txt 3rdswitchmyorg.txt/;
for my $file (@flist) {
    $file =~ /(router|switch)/i or next;
    my $rs = uc $1;
    $file =~ /(\d+)(st|nd|rd)/i or next;
    my $pos = uc "$1$2";
    print "$pos.$rs.MY.ORG.TXT\n";
}