Your program is aligned along the lines of

1. Get input
2. Calculate result
3. Output result

A sub is identical. It takes in inputs in the form of arguments, does something, and returns some outputs.

You could move your entire code into a sub, but 1) you wouldn't learn anything, and 2) you want subs to do specific tasks, so you want to separate the keyboard input from the processing from displaying the result:

use warnings; 
use strict;

print "Enter the number you would like to see in the sequence: ";
my $number = <STDIN>;

print(fibonacci($number), "\n");
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Just move the rest into the sub, get the number from the argument list and return the result.