The is_const code doesn't seem to detect undef as a constant, although Devel::Peek reports it as READONLY 
perl -w -le 'sub is_const { !eval { ($_[0]) = $_[0]; 1; } }
 print "undef is ",(is_const(undef)?"":"not")," a constant"'
undef is not a constant

perl -MDevel::Peek -le 'Dump(undef)'
SV = NULL(0x0) at 0x8120d38
  REFCNT = 2147483622
  FLAGS = (READONLY)

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In reply to Re: Re: Detecting constant arguments passed to subroutines by dcd
in thread Detecting constant arguments passed to subroutines by dcd

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