not behaves differently in pre-5.6.0 than in 5.6.0.

# under 5.005_02
  DB<1> x not(1) || 2
0  ''
  DB<2> x not 1 || 2 
0  ''

# under 5.6.0
  DB<1> x not(1) || 2
0  2
  DB<2> x not 1 || 2
0  ''
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This is because in 5.6.0 they decided "if it looks like a function, it is a function". Because, you see, in 5.005_02, the code not(1) || 2 is just like not +(1) || 2 -- which means that (1) is not an "argument" to not, since not isn't a function. In 5.6.0, they decided that this looked too odd, and therefore, if it looks like a function, it is a function.

japhy -- Perl and Regex Hacker