Just following up with the original anon-a-monk's original example...

my $i = 1;
sub f {$i += 1; $_[0] * $i}
print f(2) + f(3);
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...to see why this is bad, substitute each call of f() in the print statement with its definition...

print do{$i += 1; 2 * $i} + do{$i += 1; 3 * $i};
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...notice that there are two assignments which refer to the same variable on the RHS, which violates our anti-EOD criteria from above.