I managed to install this package via my linux package manager along with the library.

Here is something to get you started from first principles:

#!/usr/bin/perl
# by bliako
# for https://perlmonks.org/?node_id=11138293
# on 01/11/2021
use strict;
use warnings;

use Graphics::Magick;

my($image, $status);
$image = Graphics::Magick->new;
$status = $image->Read('a.jpg');

my $W = $image->Get('width');
my $H = $image->Get('height');
my $maxW = 800;
my $maxH = 200;

my ($newW, $newH, $factor) = trans($W, $H, $maxW, $maxH);
my $AR = $W / $H;
my $newAR = $newW / $newH;

print "($W x $H) => ($newW x $newH)\n";
print "original aspect ratio: $AR, new aspect ratio:$newAR\n";
$image->Resize(width => $newW, height => $newH);
$image->write("resized.jpg");

#testme();

# given original dimensions and new max dimensions
# it returns (newW, newH, resize_factor)
sub trans {
  my ($W, $H, $maxW, $maxH) = @_;

  my ($arW, $arH);

  my $factor = ($arW=($W/$maxW)) < ($arH=($H/$maxH)) ? $arH : $arW;

  my $newW = int($W/$factor);
  my $newH = int($H/$factor);
  return ($newW, $newH, $factor);
}

# this does a brute-force testing on sizes 10-1000
# for 800x200 max dim
sub testme {
  my $maxW = 800;
  my $maxH = 200;
  for my $W (map { $_*10 } 1..100){
    for my $H (map { $_*10 } 1..100){
      my ($newW, $newH, $factor) = trans($W, $H, $maxW, $maxH);
      my $AR = $W / $H;
      my $newAR = $newW / $newH;
      die "failed test: aspect ratios not the same ($AR -> $newAR) for
+ ($W x $H) => ($newW x $newH)"
        if ($AR-$newAR) > 10E-1;
      die "failed test: new width exceeds maximum for ($W x $H) => ($n
+ewW x $newH)"
        if $newW > $maxW;
      die "failed test: new height ($newH) exceeds maximum for ($W x $
+H) => ($newW x $newH)"
        if $newH > $maxH;
    }
  }
}
[download]

Output:

(2560 x 1536) => (333 x 200)
original aspect ratio: 1.66666666666667, new aspect ratio:1.665
[download]

bw, bliako