When I add use utf8; and correctly set the encoding of the output, it seems to work:

#!/usr/bin/perl
use strict;
use warnings;
use utf8;
use open OUT => ':encoding(UTF-8)', ':std';

use POSIX qw(strftime);

my $string = 'hailed an über ‘cab’ on ';
my @t = (0, 0, 0, 23, 5, 2020, 4);
my $nbsp = chr 160;

print $string . strftime( '%d/%b',  @t), "\n";
print $string . strftime( "%d$nbsp%b",  @t), "\n";
print $string . strftime("‘%d$nbsp%b’", @t), "\n";
print $string . strftime('‘%d %b’', @t), "\n";
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Update: I used the $nbsp here, as PerlMonk replaces the non-breakable space with a normal ASCII space, but it works with the nbsp character directly in the script, too.