Tricky! But there's a way (and probably more than one). do returns the value of the last evaluation in the file; this means we can stick __PACKAGE__ at the end of the file and then grab this value with do.

In do-test2.pl:

#!/usr/bin/perl
use strict;
use warnings;

while (<subdir/*.pl>) {
    my $pkg = do $_;
    print "Loaded package $pkg\n";
    {
        no strict 'refs';
        &{"${pkg}::spew"}();
    }
}
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In subdir/foo.pl:

package Foo;

print "Executing foo.pl\n";

sub spew {
    print "This is package Foo\n";
}

__PACKAGE__
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subdir/bar.pl is similar to subdir/foo.pl, as above.

Output of do-test2.pl:

Executing bar.pl
Loaded package Bar
This is package Bar
Executing foo.pl
Loaded package Foo
This is package Foo
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Dynamic.