Eww. Don't loop over a hash to find an element:

foreach $key (keys %UIDS) {
  if ($key eq $name) {
    $uid=$UIDS{$key};
     $line=join ':',$name,$passwd,$uid,$gid,$gcos,$dir,$shell;
    print TMP $line;
  }
  else {
     $line=join ':',$name,$passwd,$uid,$gid,$gcos,$dir,$shell;
    print TMP $line;
  }
}
[download]

Use the exists() function like you did before:

$uid = $UIDS{$name} if exists $UIDS{$name};
print TMP join ':', $name, $passwd, $uid, $gid, $gcos, $dir, $shell;
[download]

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