I'll suggest a couple possibilities:

1. Embed the results of time() into the file name. It's an integer, so comparisons are simplified.

2. Use the age function and let it decide how old your files are:

#
# the 'magic' of -x functions in Perl
# -s gives size of $file in bytes, -M gives age in days.
#
    my $size =-s $_;
    my $age =-M $_;
    $size /= 1048576.0;
    $total += $size;
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