I found using Imager and Imager::File::JPEG the easiest solution. Works on Ubuntu, and I assume other Linux flavors, not on Windows

here's a simple script that iterates the .jpg files in a directory and resizes them.

use strict;
use warnings;
use Imager;
use Imager::File::JPEG;
my $folder = shift;

my $dir = "/media/mark/My Passport/archive"; #starting point
$dir = $dir . '/'. $folder; #target subdirectory
chdir($dir);

my $dh;
opendir($dh, $dir);
while(readdir($dh)){
    my $fn = $_;
    next unless $fn=~ m/\.jpg$/;
    my $newimg;
    print "checking $fn\n";
    my $img=Imager->new(file=>$fn,type=>'jpeg');
    my ($height, $width);
    $height = $img->getheight;
    $width = $img->getwidth;
    my $orient = 'Portrait'; #default
    if ($height < $width){
        $orient = 'Landscape';
    }
    
    print "processing $fn height->$height width=$width orient = $orien
+t\n";
    if($orient eq 'Portrait' && $height < 800){
        $newimg = $img->scale(ypixels=>900);
    }
    elsif($orient eq 'Landscape' && $width < 1200){
        next if ($height / $width) >= 0.75;
        $newimg = $img->scale(xpixels=>1200);
    }
    else{ #dimensions OK, nothing to do
        next;
    }
    $height = $newimg->getheight;
    $width = $newimg->getwidth;
    print "saving $fn orientation->$orient, width->$width height->$hei
+ght\n";
    $newimg->write(file=>$fn, type=>'jpeg',jpegquality=>100)
                or die "cannot save altered image $img->errstr";      
+      
}
[download]