If I understand correctly what you want, then you may exploit the possibility of putting code into @INC (see perldoc -f require):

#!/usr/bin/perl 

use strict;
use warnings;

use lib sub {
    my $file=pop;
    return undef unless $file eq 'Foo/Bar.pm';
    open my $fh, '<', \<<".EOM" or die $!;
package Foo::Bar;

use strict;
use warnings;

sub new {
    "Hello World!\n",
      "this is ", __PACKAGE__, "\n";
}

1;
.EOM

    $fh;
};

use Foo::Bar;
print Foo::Bar->new;

__END__
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