if( $i == 0 ) {
    for my $w ( @Nc ) {
    }
}
else {
    for my $w ( @Nc ) {
    }
}

##</code><code>##

for my $w ( @Nc ) {
    if( $i == 0 ) {
        # First pass actions
    }
    else {
        # Subsequent pass actions.
    }
}