while (defined($_ = <>) and $_ ne "quit\n") {
    ...

##</code><code>##

    my @dupes = grep $lines{$_} > 1, keys %lines;

##</code><code>##

    % perl -ne 'END { print grep $s{$_}>1, keys %s } last if $_ eq "quit\n"; $s{$_}++' file