The 70mb it slurps is due to the @x array alone; I added a getc() to be able to check. I also added a third method - with interesting results.

#!/usr/bin/perl -w
use strict;
use Benchmark qw(cmpthese);

my @x = map join(',', split(//, rand(10000))), 0..80000;
my %y;

print "Check memory now.", $/;
getc();

cmpthese(20, {
        map   => sub { undef %y; %y = map(($_ => undef), @x) }
        for   => sub { undef %y; @y{$_} = undef for @x; },
        slice => sub { undef %y; @y{@x} = undef },
});

print scalar %y,$/;

__END__
Check memory now.

Benchmark: timing 20 iterations of for, map, slice...
       for: 16 wallclock secs (15.47 usr +  0.27 sys = 15.74 CPU) @  1
+.27/s (n=20)
       map: 28 wallclock secs (26.67 usr +  0.30 sys = 26.97 CPU) @  0
+.74/s (n=20)
     slice: 14 wallclock secs (13.54 usr +  0.24 sys = 13.78 CPU) @  1
+.45/s (n=20)
         Rate   map   for slice
map   0.742/s    --  -42%  -49%
for    1.27/s   71%    --  -12%
slice  1.45/s   96%   14%    --
59853
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Update: had mixed up the for and slice labels. for doesn't win.

Makeshifts last the longest.