The difference is in the magic that makes Perl, well, Perl.

Scalars can be numeric or "strings". When you store an integer in a scalar it's stored as an IV. When you use that scalar in string context it gets stringified. The scalar is modified behind the scenes to be a PVIV with pointers to an IV and PV.

#!/usr/bin/perl

use strict;
use warnings;
use Devel::Peek;

my @a = (1) x 1024;
my @b = (1) x 1024;

open my $fh, "> /dev/null" or die;
print $fh $_, "\n" for @a;
print $fh "$_\n" for @b;
close $fh;

Dump $a[0];
Dump $b[0];
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Update: Fixed wording of fourth sentence.