I can come up with three possible solutions to your issue. First, you could capture the number and then test it:

if (/^(\d+\.\d+\t(\d+)\.\d+)/ and $2 > 50 and $2 < 250) {

Second, rather than simply using \d+, you could use a series of character classes linked together with alternators:

/^(\d+\.\d+\t(?:5[1-9]|[6-9]\d|1\d{2}|2[0-4]\d|250)\.\d+)/

Lastly, you could incorporate Perl code into your regular expression, as described in A bit of magic: executing Perl code in a regular expression. Reading material is available on all these options in perlretut. Frankly, the first option is going to be the most clear when you look at your code again next year.