You seem to be a lot more comfortable with shell programming than Perl programming. I think you could do all of what you want in the shell like so:

for file in $outdir/*postpaid*
do
    sort -t '&' -k 2 < $file > $file.ok
done
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You can also do it all in Perl, and there are many ways to accomplish that. This is probably one of them:

opendir(INDIR, $outdir) or die "Can't opendir $outdir: $!\n";
while (my $infile = readdir(INDIR)) {
    next if $infile !~ /postpaid/;

    open my $infile_fh, '<', "$outdir/$infile"
        or die "Can't read $infile: $!\n";
    my @lines = <$infile_fh>;
    close $infile_fh;

    open my $outfile_fh, '>', "$outdir/$infile.ok"
        or die "Can't write $infile.ok: $!\n";
    print $outfile_fh
          sort { ($a1) = ($a =~ /^\d+\&(\d{8})\&/);
                 ($b1) = ($b =~ /^\d+\&(\d{8})\&/);
                 $a1 <=> $b1 } @lines;
    close $outfile_fh;
}
closedir(INDIR);
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