You have use strict; so you must declare variables or the error you see will be thrown. If you've copied/paste this without understanding I suggest you read and understand strict. An alternative would be to declare $i within the for loop.

"How to merge these three lines"

One way to write this:

use strict;
use warnings;

my $number=8;

for( my $i=0; $i<20; $i++ ){
    print "present value of the number is: $number\n";
    $number++;
}

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If you are new to perl the following links will likely be of interest:

In reply to Re: usage of "my" keyword by marto
in thread usage of "my" keyword by ravi45722

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