...assuming @files contains the list of filenames in the CWD:

my @sorted = sort { -s $a <=> -s $b } @files;
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Or for a more complete example...

use strict;
use warnings;
{
    opendir (my $dir, '.') or die "Can't open the directory.\n$!";
    local $, = "\n";
    print sort { -s $a <=> -s $b } grep -f, readdir($dir);
    closedir $dir;
}
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See perlfunc under the -X functions to understand what -s is.