use strict;
use warnings;
use File::Slurp qw/ read_file write_file /;

$ARGV[0] or die "\nUsage: $0 <File>\n\n";

my $userlistContents = read_file $ARGV[0];

for ( grep /\S/, <DATA> ) {
	my ( $before, $after ) = split;
	$userlistContents =~ s/\b$before\b/$after/g;
}

write_file "changed_$ARGV[0]", $userlistContents;

__DATA__
JOE	JOHN
FRED	FREDERICK
SAM	SAMANTHA