$ perl -Mstrict -Mwarnings -E '
my ($string, $offset, $length) = qw{ATATTTATATTAT 0 3};
$string =~ /^(.{$offset})(.{$length})(.*)$/;
say "Extracted: ", $2 // "";
say "Remainder: ", ($1 // "") . ($3 // "");
'
Extracted: ATA
Remainder: TTTATATTAT