Bug 19991: use Modern::Perl in OPAC perl scripts

Bug 19991: use Modern::Perl in OPAC perl scripts

Test Case:
Check the following files have been updated from
use strict;
use warnings;
to
use Modern::Perl;

errors/400.pl
errors/401.pl
errors/402.pl
errors/403.pl
errors/404.pl
errors/500.pl
opac-account-pay-paypal-return.pl
opac-alert-subscribe.pl
opac-authorities-home.pl
opac-authoritiesdetail.pl
opac-browser.pl
opac-ics.pl
opac-image.pl
opac-imageviewer.pl
opac-messaging.pl
opac-modrequest-suspend.pl
opac-modrequest.pl
opac-mymessages.pl
opac-overdrive-search.pl
opac-passwd.pl
opac-patron-image.pl
opac-privacy.pl
opac-ratings-ajax.pl
opac-ratings.pl
opac-readingrecord.pl
opac-renew.pl
opac-sendshelf.pl
opac-serial-issues.pl
opac-showreviews.pl
opac-suggestions.pl
opac-tags_subject.pl
opac-topissues.pl
opac-user.pl
sco/help.pl
sco/printslip.pl
sco/sco-patron-image.pl
svc/overdrive_proxy
svc/suggestion
unapi

Signed-off-by: Maryse Simard <maryse.simard@inlibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>