List::Util is a core module (ships with Perl). It provides the mesh() function, which can be used as follows:

my @doubled = mesh \@array, \@array;
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It turns out to be really quick.

#!/usr/bin/env perl

use v5.36;

use strict;
use warnings;
use Benchmark qw(cmpthese);
use List::Util qw(mesh);

my @array = ('a' .. 'z');

cmpthese(
    -5,
    {
        mapped => \&mapped,
        meshed => \&meshed,
    },
);

sub mapped {
    return [map {$_,$_} @array];
}

sub meshed {
    return [mesh \@array, \@array];
}
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The output:

           Rate mapped meshed
mapped 111262/s     --   -51%
meshed 226692/s   104%     --
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When I bump it up to 676 elements ('aa' .. 'zz'), the comparison is similar:

         Rate mapped meshed
mapped 4157/s     --   -50%
meshed 8262/s    99%     --
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With 11756 elements ('aaa' .. 'zzz') the ratio holds.