you

donīt use strict;

donīt use the two argument form of bless();

donīt get a reference to self in your method

use the bareword self where you mean $self in new()

That is what causes the wrong behavior of your class.
Compare to this code (that works as intended):

use strict;

my $p = Foo->new();
$p->bkg_img("whatever");
$p->print_html;


package Foo;

sub new
{
    my $class = shift;
    my $self  = {};

    $self->{BKG_IMAGE} = "image";

    bless($self, $class);
    return $self;
}

sub bkg_img 
{
    my $self = shift;
    $self->{BKG_IMAGE} = $_[0];
}

sub print_html 
{
    my $self = shift;

    print <<"HTML";
    $self->{BKG_IMAGE}
    HTML
}
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