Here's another approach to changing how expressions are parsed: start with strings and eval them after you've turned them into the Perl equivalent. The dechain sub turns a string into a series of and-ed expressions, which you could then eval.

my $op = qr/[!=<>]+/;
sub dechain {
  my @terms;
  while ($_[0] =~ /(.+?)($op)(?=((?:(?!$op).)+))/g) {
    push @terms, join '', $1, $2, $3;
  }
  join ' and ', @terms;
}

print dechain('3 < 4 < 5'), "\n";
print dechain('$x < $y <= $z'), "\n";
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