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$ perl -Mstrict -Mwarnings -E '
my ($string, $offset, $length) = qw{ATATTTATATTAT 0 3};
...
'
Extracted: ATA
Remainder: TTTATATTAT

sub split_string ($$$) {
    my ($string, $offset, $length) = @_;
...

    return (($1 // ''), ($2 // ''), ($3 // ''));
}

my ($left, $extract, $right) = split_string $string, $offset, $length;
my $remainder = $left . $right;

#!/usr/bin/env perl

...
        " joined[@{[$left . $right]}]";
    say '-' x 72;
}

$ pm_substr_and_remainder.pl
string[ATATTTATATTAT] offset[0] length[3]
...
string[1234567890] offset[5] length[0]
left[12345] extract[] right[67890] joined[1234567890]
----------------------------------------------------------------------
+--