When does the difference between a string and a number matter in Perl 5?

Question

If a string in Perl 5 passes looks_like_number, it might as well be a number. For instance,

my $s = \"10\" + 5;

results in $s

Answer 1

  DB<1> sub is_num { my $x = shift; "$x " ~~ $x }

  DB<2> print is_num(123)
1
  DB<3> print is_num('123')

  DB<4>