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

An equivalent number and string behave differently in hash keys -- or, more generally, any time we stringify a largish number:

my (%g, %h);
$g{ 1234000000000000 } = undef;  # '1.234e+015'       => undef
$h{'1234000000000000'} = undef;  # '1234000000000000' => undef

Note that we are still within the range where Perl can store the number precisely:

> perl -e 'printf qq{%.f\n}, 1234000000000000 + $_ for +1, 0, -1'
1234000000000001
1234000000000000
1233999999999999