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;
    }
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