Since the input data appear to be delimitted by the "=", using a split to generate the sort key might be more robust:

#!/usr/bin/perl

use warnings;
use strict;

my @values = (
    '123=more digits',
    '04=critical',
    '02=informational',
    '01=unknown',
    '10=test',
    '03=warning',
    '08=foo',
);

@values =
    map  { $_->[1] }
    sort { $a->[0] <=> $b->[0] }
    map  { [ ( split /=/ )[0], $_ ] } 
        @values;

print "@values\n";
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output = 01=unknown 02=informational 03=warning 04=critical 08=foo 10=test 123=more digits