A not so simple way of doing this in Imager:

#!perl -w
use strict;
use Imager;

my $from = shift;

my $to = shift;

my $in = shift;

my $out = shift
  or die "Usage: $0 fromcolor tocolor inimage outimage\n";

my $from_color = Imager::Color->new($from)
  or die "Cannot convert fromcolor $from into a color: ", Imager->errs
+tr, "\n";

my $to_color = Imager::Color->new($to)
  or die "Cannot convert tocolor $to into a color: ", Imager->errstr, 
+"\n";

my $img = Imager->new;
$img->read(file=>$in)
  or die "Cannot read image $in: ", $img->errstr, "\n";

my $result = replace_color($img, $from_color, $to_color)
  or die "Cannot replace colors: ", Imager->errstr, "\n";

$result->write(file=>$out)
  or die "Cannot write image $out: ", $result->errstr, "\n";

sub replace_color {
  my ($img, $from_color, $to_color) = @_;

  my ($from_red, $from_green, $from_blue) = $from_color->rgba;
  my ($to_red, $to_green, $to_blue) = $to_color->rgba;
  my $rpnexpr = <<'EOS';
x y getp1 !pix
@pix red from_red eq
@pix green from_green eq
@pix blue from_blue eq
and and
to_red to_green to_blue rgb @pix ifp
EOS
  my %constants =
    (
     from_red => $from_red,
     from_green => $from_green,
     from_blue => $from_blue,
     to_red => $to_red,
     to_green => $to_green,
     to_blue => $to_blue,
    );
  return Imager::transform2({ rpnexpr => $rpnexpr,
                  constants => \%constants },
                $img);
}
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