If it's not a huge file, you could read it into memory and use map to iterate over all the lines:

#!/usr/bin/perl -w
use strict;

my $ifile = shift || die "No filename provided\n";
open IFILE, '<', "$ifile"    || die "Couldn't open file: $!";
open OFILE, '+>', 'new-data' || die "Couldn't open outfile: $!";
chomp(my @lines = <IFILE>);
close IFILE;

my @matches = map { /^@.*mail(\d+).*\z/? $1 <= 8? $1: ( ): $_ } @lines
+;

map { print OFILE "$_\n" } @matches;
close OFILE;
[download]

But I don't see anything wrong with the way you did it, except that I would recommend error-checking for the case where a filename isn't passed to the program.

Update:  Fixed to take skip saving anything in the case where the captured pattern isn't <= 8.

Update 2:  Cleaned up syntax further.