use strict;
use warnings;
use 5.010;

my @filelist = glob('c:\data\perl\bin\*.bat');

{local $,="\n"; say 'Unsorted', @filelist;}
say;

my @sorted = sort by_last_mod @filelist;

{local $,="\n"; say 'Sorted', @sorted};

sub by_last_mod {
    my $adate = ( stat($a) )[9];
    my $bdate = ( stat($b) )[9];
    return $adate <=> $bdate;
}
[download]

Unsorted
c:\data\perl\bin\ap-iis-config.bat
c:\data\perl\bin\ap-update-html.bat
c:\data\perl\bin\ap-user-guide.bat
c:\data\perl\bin\c2ph.bat
(...)

Sorted
c:\data\perl\bin\latexmk.bat
c:\data\perl\bin\dprofpp.bat
c:\data\perl\bin\exetype.bat
c:\data\perl\bin\perlglob.bat
(...)
[download]

How do you populate the @filelist array?    Are you using readdir?

foreach $file (sort by_last_mod @filelist) {push (@sorted,$file);} could more simply be written as: push @sorted, sort by_last_mod @filelist;.

Yes I am using readdir.

I checked that @filelist is correct (really contains the file names), and that it is not already sorted (LOL);

After the sort, I check @sorted and see that it is exactly the same then @filelist. No sort at all was done.

I also used File::Stat, same result :(

If you are using readdir then you have to prepend the directory name to the file name when you do the stat, for example:

opendir DH, $dir or die "Cannot opendir '$dir' $!";

my @filelist = readdir DH;

my @sorted = sort by_last_mod @filelist;

sub by_last_mod {
    my $adate = ( stat "$dir/$a" )[ 9 ];
    my $bdate = ( stat "$dir/$b" )[ 9 ];
    return $adate <=> $bdate;
    }
[download]