It doesn't really just make a guess. It grabs all the numeric value it can find from the front of the string, and truncates anything past that. You can see the effect in action:

my @strings = qw(
    2,300
    2.54xyz123
    2e32abc3
    2.34,23
);

for (@strings) {
    print "$_ => ", 0+$_, "\n";
}
[download]

This prints:

2,300 => 2
2.54xyz123 => 2.54
2e32abc3 => 2e+32
2.34,23 => 2.34
[download]