push @missing_ids, $idslist[ $_ ] +1 .. $idslist[ $_+1 ] -1 for 0 .. $#idslist-1;

##</code><code>##

for ( 0 .. $#idslist-1 ) {
    push @missing_ids, $idslist[ $_ ] +1 .. $idslist[ $_+1 ] -1;
}