Email Sent By Perl (Net::SMTP) Script Dropped Silently by Gmail - perl

Question

I have a Perl script that sends simple HTML emails to users with status updates and a link to more information. I'm creating mail with Net::SMTP and sending mail using smtp-relay.gmail.com. It has been working great for years.
In the last month, the emails stopped appearing - just gone with no errors and not in SPAM. Hours of troubleshooting later, I've narrowed the problem down to Gmail silently dropping emails that contain my specific URL.
http://DOMAIN/cgi-bin/requests/single_request.pl?requestid=111111
I changed one character and POOF! working again.
http://DOMAIN/cgi-bin/requests/single-request.pl?requestid=111111
I know this is a vague question, but what is going on? The workaround is fine, but I sure didn't learn anything about the root cause.
#!c:\strawberry\perl\bin -w
use strict;
use warnings;
use Net::SMTP;
use Net::Config;
# use this function to send email
#
# example:
#
# send_mail("RECIPIENT\#DOMAIN", "SUBJECT HERE", "BODY HERE");
sub send_mail{
my $recipient = shift;
my $subject = shift;
my $body = shift;
# connect to an SMTP server
my $smtp = Net::SMTP->new("smtp-relay.gmail.com", Debug => 0, Timeout => 30, Hello => 'REDACTED') or die "SMTP Connection Failed: smtp-relay.gmail.com";
# sender's address here
$smtp->mail('REDACTED');
# recipient"s address
$smtp->to($recipient);
# Start the mail
$smtp->data();
# Send the header.
$smtp->datasend("MIME-Version: 1.0\n");
$smtp->datasend("Content-Type: text/html; charset=\"UTF-8\" \n");
$smtp->datasend("To: $recipient . \n");
$smtp->datasend("From: REDACTED\n");
$smtp->datasend("Reply-to: REDACTED\n");
$smtp->datasend("Subject: $subject \n");
$smtp->datasend("\n");
# Send the body.
$smtp->datasend($body);
# Finish sending the mail
$smtp->dataend();
# Close the SMTP connection
$smtp->quit();
}
1;