From perldoc -f -M

-M  Script start time minus file modification time, in days.

Ah, thanks. That makes things perfectly clear.

$PM = "Perl Monk's";
$MCF = "Most Clueless Friar Abbot Bishop Pontiff Deacon Curate Priest Vicar";
$nysus = $PM . ' ' . $MCF;
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BTW, perldoc is wrong, it's not "script start time", it's $^T as shown by the following:

#!/usr/bin/perl -l

# https://perlmonks.org/?node_id=1230174

use strict;
use warnings;
use Path::Tiny;

my $file = 'tmp.1230174';

sleep 2;
path($file)->spew("$file\n");
sleep 2;

print -M $file;
$^T = time;
print -M $file;
[download]

Outputs:

-2.31481481481481e-05
2.31481481481481e-05
[download]

So you can play games with "file age" if you want to.