My grasp of postfix for is sometimes a little fuzzy, but if you re-write

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

for ( 0 .. $#idslist-1 ) {
    push @missing_ids, $idslist[ $_ ] +1 .. $idslist[ $_+1 ] -1;
}
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I could see why @missing_ids is bigger. For every element of @idslist, you are pushing consecutive numbers/letters between $idslist[ $_ ] +1 and $idslist[ $_+1 ] -1, which could be very large indeed. How's that?