#1:
perl -E"my @aa = 1..20_000_000; my @bb = 1..20_000_000;" #error! out of memory!

#2:
perl -E"my @aa = 1..20_000_000; my @bb = @aa;" #works well
#3:
perl -e"my @cc = 1..40_000_000" #error! out of memory

#4:
perl -E"my @cc; for(my $i = 1; $i<40_000_001; $i++){ push @cc, $i;} print scalar @cc"  #works well.

#5:
perl -e"push @cc,$_ for 1..40_000_000; print scalar @cc;" works well