# $infile is assumed to be the file you are reading...

# I'm assuming you know what you want to put in the out
# file name and how to get it...
my $otherinfo = &getotherinfo;

# the \d{2} is the day, make it \d{1,2} if you want to
# allow weblog.2Jul as well as weblog.23Jul
#
# the \w{3} catches the month abbreviation
(my $outfile = $infile) =~ s/\.\d{2}\w{3}/-$otherinfo/;

# do this if you want to use an absolute pathname...
$outfile = $pathname . $outfile;

open(OUTFILE, "> $outfile");