just found that B::Xref lists sub-calls and(!) sub-definitions:

 perl -MO=Xref tst.pl |grep foo
tst.pl syntax OK
      &foo              s1
      &foo              &1, &4
  Subroutine foo
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so just needing to parse¹ and compare the output.

But I agree that the dynamic nature of Perl makes such static parsing look strange.

1) especially the raw output could be easily processed:

$ perl -MO=Xref,-r tst.pl |grep foo
tst.pl           (definitions)     1 main            & foo            
+  subdef
tst.pl syntax OK
tst.pl           (main)           1 main            & foo             
+ subused
tst.pl           (main)           4 main            & foo             
+ subused
tst.pl           foo              3 main            $ _               
+ used
$ cat tst.pl 
foo();
 
sub foo { print }
foo();
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