For instance in a previous assignment, I really wanted to take a user input and force it to be a floating point number not a string (for math equations), but I could only find a way to take user input and make it an integer "int(<STDIN>)", but what if they enter a number with a decimal? Why is it basically impossible to easily force user input to be a number if it's not a whole number?

A scalar variable in perl has various internal representations: integer, string, number - of which "number" also handles decimals. Conversion from string to numbers and vice versa happens internally according to some rules. You find all pertaining to that in the perldata perl manual page.

If you input a decimal number, chances are great that you don't enter the binary representation of that number, but rather a string representation, e.g. the decimal number "1.23" consists of 4 chars (or bytes, equivalent in this case); perl will convert that to float internally and store that value in the number slot of the variable, as soon as you treat that value in a numeric context - as in math.