I think you'll want DBL_MAX and DBL_MIN (or LDBL_MAX and LDBL_MIN if $Config{uselongdouble}). And LDBL_MIN isn't the minimum value, it's the minimum possible normal value greater than 0. "Normal" here means a number with the full mantissa's worth of precision. Most implementations allow "subnormal" numbers that can get smaller. For example:

$ perl -MPOSIX=DBL_MIN -wle'print DBL_MIN()'
2.2250738585072e-308
$ perl -MPOSIX=DBL_MIN -wle'print DBL_MIN()/2**52'
4.94065645841247e-324
$ perl -MPOSIX=DBL_MIN -wle'print DBL_MIN()/2**53'
0
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