Thank you, this is probably the best way of looking at it! I took your idea and expanded on it, below. It seems that the answer to my original question is No, there isn't any magic going on with for (split ...), it's just that while it may be a memory hog, split is still pretty fast - that's what threw me off initially.

#!/usr/bin/env perl
use warnings;
use 5.014;

my $cnt = 5000000;
system($^X,'-sE',<<'_END_','--',"-cnt=$cnt");
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 4340
    $x = "abc\n" x $cnt;
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 23936
    @y = ("abc") x $cnt;
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 455604
_END_
say "---";
system($^X,'-sE',<<'_END_','--',"-cnt=$cnt");
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 4544
    $x = "abc\n" x $cnt;
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 23972
    @y = split /\n/, $x;
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 423544
_END_
say "---";
system($^X,'-sE',<<'_END_','--',"-cnt=$cnt");
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 4540
    $x = "abc\n" x $cnt;
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 23964
    for (split /\n/, $x) {
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 457512
    last }
_END_
say "---";
system($^X,'-sE',<<'_END_','--',"-cnt=$cnt");
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 4336
    $x = "abc\n" x $cnt;
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 23932
    open my $fh, '<', \$x or die $!;
    while (<$fh>) {
    say `ps -orss $$`=~s/\s+/ /gr;  # RSS 24000
    last }
_END_
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