If you eval a string containing a named sub definition, it gets installed into the current package:

c:\test>perl -E"eval 'sub Foo{ return 12345 }'; print Foo();"
12345
[download]

If you want to create an anonymous sub and get a reference to it into a variable:

c:\test>perl -E"$f = eval 'sub { return 12345 }'; print $f->();"
12345

c:\test>perl -E"$f = eval 'sub { return "bar" }'; print $f->();"
bar
[download]

That works also.