not sure why you assign to $_ here ...

>  $_ = [$1, $2, $_]

But I felt tempted to try a variation with named captures for clarity. (tests borrowed from Hauke's post)

use strict;
use warnings;
use Data::Dump;
use Test::More;

my @in  =
  ( "2bc", "1", "12c", "12", "21", "1", "1a", "2", "2", "2",
    "2", "3", "35", "31", "2b", "4", "42", "5", "51", "2ac",
    "52", "6", "7"
  );



my @exp =
  ( "1", "2", "3", "4", "5", "6", "7", "12", "21", "31",
    "35","42", "51", "52", "1a", "2ac", "2b", "2bc", "12c",
  );


my %in;

@in{@in}=();                            # unique


my @got =
  map { $_->{orig} }
  sort { $a->{alpha} cmp $b->{alpha} || $a->{num} <=> $b->{num} }
  map { /(?<num>\d+)(?<alpha>[a-z]*)/; { orig => $_, %+ } }
  keys %in;


is_deeply(\@exp,\@got) or ddx [\@exp,\@got];
done_testing;
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