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

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

open FH,">output.txt" or die "can't open file for writing:$!";

for my $string (@input) {

print FH  $1 if($string=~/^UID:\s*(\w+)/);

}

close FH;