I made a few minor adjustments, but it seems OK otherwise.

#!/usr/bin/perl

use strict;
use warnings;
use CGI::Session;

sub login {
    my ($dbh) = @_;

    my ($email)    = $dbh->quote( my $CGI->param('email') || undef );
    my ($password) = $dbh->quote( $CGI->param('password') || undef );
    my ($loginSessionID) = $CGI->param('loginSessionID') || undef;

    eval {
        do {
            my $tmp = "SELECT\n
            id\nFROM\n
            users\nWHERE\n
            LOWER(email) =
            LOWER($email)\n
            AND password = $password\n";

            my $sth = &return_query( $dbh, $tmp );
            my (@row) = $sth->fetchrow_array;
            my %result;
            $result{'userID'} = undef;
            if (@row) {
                $result{'userID'} = $row[0];
                CGI::Session->name(
                    my $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{'n
+ame'}
                      || 'CGISESSID' );
                my $SESSION = CGI::Session->load(
                    'driver:MySQL',
                    $CGI || $loginSessionID,
                    {
                        DataSource => join( ':',
                            $CONFIG->{'mysql'}{'DSN'},
                            $CONFIG->{'mysql'}{'Collection'} ),
                        TableName   => 'sessions',
                        IdColName   => 'id',
                        DataColName => 'data',
                        Handle      => $dbh,
                    }
                ) || die( CGI::Session->errstr ) . "\n";

                if ( $SESSION->is_expired ) {
                    $SESSION->delete();
                    $SESSION->flush();
                }
                if ( $SESSION->is_empty ) {
                    $SESSION = CGI::Session->new();
                }
                $result{loginSessionID} = $SESSION->id();
                $SESSION->param( 'userID', $result{'userID'} );
                $SESSION->expire(
                    $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{expir
+e}
                      || '+30m' );
                my %COOKIES;
                $COOKIES{'loginSession'} = $CGI->cookie(
                    -name =>
                      $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{'na
+me'},
                    -value => $SESSION->id(),
                    -expires =>
                      $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{'ex
+pire'}
                      || "+30m",
                    -path =>
                      $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{'pa
+th'}
                      || "/",
                    -domain =>
                      $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{'do
+main'}
                      || "",
                    -secure =>
                      $CONFIG->{'userSession'}{'cookies'}{'LOGIN'}{'se
+cure'}
                      || 0,
                );
                $SESSION->flush();
            }
        };
        my %result;
        if ($@) {
            push @{ $result{'error'}; }, $@;
        }
        return encode_json( \%result );
    }
}
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