Since BEGIN-phase processing is when subs get defined, and the processing goes top-to-bottom, put your BEGIN block after the subs. 
use warnings;
use strict;

BEGIN {
  print "Before: $_\n" for grep /f/, keys %{main::};
}

sub foo {
  print "I am defined\n";
}

BEGIN {
  print "After: $_\n" for grep /f/, keys %{main::};
}

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In reply to Re^3: Detecting an imported function (exclude time) by Roy Johnson
in thread Detecting an imported function by Ovid

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