$frame->DragAcceptFiles(1);
EVT_DROP_FILES( $frame, \&ondrop );
EVT_BUTTON( $frame, $ButExtract, \&onExtract );
## its like
## EVT_BUTTON( $parent, $child, \&callback );
## EVT_BUTTON( $parent, $child->GetId, \&callback );
...
sub ondrop {
    my( $frame, $wxDropFilesEvent ) = @_;
    my @files = $wxDropFilesEvent->GetFiles;
    $frame->{you_chose_these_files} = \@files;
    ## extract( $files[0] ); ## or whatever
}

sub onExtract {
    my( $frame, $extractBut ) = @_;
    my( $firstFile ) = @{    $frame->{you_chose_these_files}   };
    extract( $firstFile );
    ...
}