my $first = 'run_this_first'; # or whatever
@filenames = sort { ($b eq $first) cmp ($a eq $first)
   || -M $b <=> -M $a } @filenames;

##</code><code>##

my %modtimes;
$modtimes{$_} = -M $_ foreach @filenames;
$modtimes{$first} = 1e99;
@filenames = sort { $modtimes{$b} <=> $modtimes{$a} } @filenames;