gistfile1.pl

#!/usr/bin/perl
use strict;
use warnings;

use Data::Dumper;

use namespace::clean;

BEGIN {
  my $symtab;
  {
    no strict 'refs';
    $symtab = \%{'main::'};
  }
  last if ( exists $symtab->{'foo'} );
  my $debugging = sub() { 0 };
  if ( exists $ENV{'TEST_DEBUGGING'} ) {
    $debugging = sub(){ 1 } if (( $ENV{'TEST_DEBUGGING'} + 0 ) > 0 );
  }
  {
    no strict 'refs';
    *{'main::foo'} = $debugging;
  }
}

print '.' . foo . '/';

if (foo) {
  print 1;
}
else {
  print 0;
}

sub bar {
  my %symtab;
  {
    no strict 'refs';
    %symtab = %{'main::'};
  }
  if ( exists $symtab{'foo'} ) {
    print 'exists';
  }
  print $symtab{'foo'}->();
  print Dumper(\%symtab );
}

bar();