Is there a standard way to do it?

No but there are many ways to do it. Here is one:

#!/usr/bin/env perl 
use strict;
use warnings;

use List::Util 'reduce';
use Path::Tiny 'path';

my @files = qw/a b c/;
my $dir = '/tmp/';

print newest($dir, @files) . "\n";

sub newest {
    my $dir = shift;
    my $newest = reduce { $a->stat->mtime > $b->stat->mtime ? $a : $b 
+}
        map { path "$dir/$_" }
        @_;
    return "$newest";
}
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I've passed the files as an array since that is much more useful (in a generic sense) than your hashref. Feel free to modify for your own purposes, of course. If your list of files is (or might be) very long then consider a transform to avoid excessive stat calls.