I didn't see any good answers to this given. I'd do:

my $x= 1+~0; $x *= 2 while $x != $x+1;
print "You can safely use integer values between ",
    -$x/2, " and ", $x/2, $/;
#   -$x, " and ", $x-1, $/;
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where you could choose to use that last line instead of the second-to-last line if you aren't paranoid like me.