Welcome. If this is a homework or exercise question it may help to post the actual challenge. Assuming that you want to prompt the user to enter one number, then subtract from that a number depending on user input value, try something like this.

#!/usr/bin/perl

use strict;
use warnings;

print "Enter a number\n";
my $number = <>;
chomp($number);

my $sendthis = '';

if ( $number ) {

    if ( $number >= 50 ) {
        $sendthis = $number - 4;
    }
    elsif ( $number >= 20 ) {
        $sendthis = $number - 3;
    }
    elsif ( $number >= 10 ) {
        $sendthis = $number - 2;
    }else{
        $sendthis = $number;
    }
}
print "You sent: $sendthis\n";
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