So how can a package return true to -->can('function') and yet it can't execute function? It seems to me that that can't happen?

Because can (in class form) does respect inheritance, while a fully specified function name (like foo::bar) does not respect inheritance. Look at the following reduced code:

package Parent;
sub foo { print "foo in parent(@_)\n"; };
sub bar { print "bar in parent(@_)\n"; };
package Child;
@ISA = qw(Parent);
sub foo { print "foo in child(@_)\n"; };
package main;

eval { Child::foo(); }; warn $@ if $@;
eval { Child->foo(); }; warn $@ if $@;
eval { Child::bar(); }; warn $@ if $@;
eval { Child->bar(); }; warn $@ if $@;
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Thank you Corion, that was very helpful. In the meantime I have also discovered the source of my confusion. I made the (wrong) assumption that simply saying

      our @EXPORT = qw(function);
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means that Spiffy would export function into package two. Actually what I need to do is declare that package one is the base class package two, like this:

      use one -base;
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The problem is that package two lives in the same file as package one, and this causes a compiler error. The whole thing can be resolved by adding:

      import one;
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after the

      @two::ISA = qw(one);
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Thus the final program, and its output is:

#!/usr/bin/perl

package one;
use Class::Spiffy -base;
our @EXPORT = qw(function);
sub function { print "Function in package one\n" }

package two;
@two::ISA = qw(one);
import one;
print STDERR "Class::Spiffy version is ", $Class::Spiffy::VERSION, "\n
+";
print STDERR "Can two function? ",two->can('function'), "\n";
function();

__END__

cd /home/henry/dev/
/usr/bin/perl -w /home/henry/dev/spiffy_bug.pl 
Class::Spiffy version is 0.15
Can two function? CODE(0x814b738)
Function in package one

Compilation finished at Tue Feb 21 10:37:35
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Thanks to all who have tried to be helpful in solving this mystery.
Best wishes,
Henry Laxen