Or

#!usr/bin/perl
use strict; 
use warnings; 
# 848903

my @strings = ("Supervisor UID: plbaerr4",
                "UID: dlsmith3",
                "Peon UID: jklmn2");

for my $string(@strings) {
    if ( $string !~ /^Supervisor \w+/ && $string =~ /UID: (\w+)/ ) {
        print "$1 \n";
    } else {
        print "No match in $string \n";
    }
}
[download]

almut's reply handles the file-writing spec; this addresses ambiguity in OP's spec: is it the "Supervisor" UID that's verbotten or any UID preceded by a title?

Updated: preceding para dropped from cut and thus not pasted. Duh.