...
open( IN, $list2 ) or die "$list2: $!";
while (<IN>) {
    next unless ( m:(.+?)#(\d+)$: );
    my ( $p, $v ) = ( $1, $2 );

    next unless ( exists( $pathname_versions{$p} ));

    if ( exists( $pathname_versions{$p}{$v} )) {
        # get here if we have a full match (path and version)
    }
    else {
        # create a sorted list of version numbers from list1:
        my @list1versions = sort {$a<=>$b} keys %{$pathname_versions{$p}};

        if ( $v < $list1versions[0] ) {
            # list2 version number is lower than _all_ matching list1 entries
        }
        elsif ( $v > $list1versions[-1] ) {
            # list2 version number is higher than _all_ matching list1 entries
        }
    }
}