You should look up substr. (Update: choroba put a more detailed explanation while I was writing this, so see below)

Alternatively, in the spirit of TMTOWDI, you also could adopt the following algorithm

1. split the words into an array characters
2. terminate if less than 4 chars in array
3. print the first 4 characters on the array
4. remove (shift the first character off the array
5. go back to 2)

my $word = 'ABCDEFGH';
my @word = split //, $word;

my $pos = 1;
while (scalar(@word) >= 4) {
   print @word[0..3]."==> starting at $pos";
   shift @word;
   $pos++;
}
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