You're probably re-declaring a variable. Here's my edit of the code, which runs cleanly and correctly:

use strict;
use warnings;
use Time::Local;

# subroutine extracts and formats date from a time stamp
sub extractDate($) {
    # extracts and formats date from a time stamp
    my ($t) = @_;
    my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)
    = localtime($t);
    $mon  += 1;
    $year += 1900;
    $mon = (length($mon) == 1) ? "0$mon" : $mon;
    $mday = (length($mday) == 1) ? "0$mday" : $mday;
    return ($mon, $mday, $year);
}

# get all data from website from 8/16/07 to present
my $start = timelocal(0,0,0,16,7,107);
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst);
($mon, $mday, $year) = extractDate($start);
print "starting at $mon/$mday/$year\n";

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time
+);
my $now = timelocal(0,0,0,$mday - 0,$mon,$year);
#$now = timelocal(0,0,0,$mday - 1,$mon,$year);
#$now = timelocal(0,0,0,3,10,105);
($mon, $mday, $year) = extractDate($now);
print "ending at $mon/$mday/$year\n";

my ($t);

for ($t = $start; $t < $now; $t += 60*60*24) {
    ($mon, $mday, $year) = extractDate($t);
    print "processing $mon/$mday/$year\n";
}
[download]