Declare your sub before the BEGIN and it should work.

No, it won’t:

18:37 >perl -Mstrict -wE "use subs 'TestMe'; BEGIN { TestMe(); } sub T
+estMe { say 'Hi!'; }"
Undefined subroutine &main::TestMe called at -e line 1.
BEGIN failed--compilation aborted at -e line 1.

18:37 >
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As Anonymous Monk pointed out above, you have to define the sub before the BEGIN block:

18:37 >perl -Mstrict -wE "sub TestMe { say 'Hi!'; } BEGIN { TestMe(); 
+}"
Hi!

18:41 >
[download]

In Perl, as in C, it is important to keep in mind the distinction between declaration and definition.