NOT because this is useful, elegant, or even extensible code: hardcoded vars, sted calculated vars; RITW (and in this case, probably a five-wheeled q'cycle). But just because embedded newlines intrigue me...

#!\usr/bin/perl
use warnings;
use strict;
use 5.016;

# 1091171 with modifications to handle strings w/embedded newlines

my @data = <DATA>;

my $source = ('-' x 50 . "\n" .  '-' x 75);
my @id = split /\n/, $source;
my ($tag, $line, $id, $offset, $end, $substr_len, $len, $range);
for $id(@id) {
    chomp $id;
    if ( $id =~ /^-{50}$/ ) {
        $len = length($id);
        $tag = "VAR1: ";
        for $range(@data)  {
            ($offset, $end) = split /-/, $range;
            chomp $offset;
            chomp $end;
            my $substr_len = ($end - $offset + 1);
                if ($offset + $substr_len <= $len) {
                    substr ($id[0], $offset - 1, $substr_len) = ('A' x
+ $substr_len);
                    say "$tag: id[0]\n\tModified line: |$id[0]|\n";
                } else {
                    say "At Ln26: \$end: $end  > \$len: $len";
                    say "Replacements attempted beyond end of string.\
+n\n";
                    next;
            }
        } $id = '';
    } else {
           $tag = "VAR2: ";
           say "\n\t\t NOW WORKING ON $tag \n";
            chomp $id[1];
            my $len = length($id[1]);
            for $range(@data) {
                my ($offset, $end) = split /-/, $range;
                chomp $offset;
                chomp $end;
                my $substr_len = ($end - $offset + 1);
                if ($offset + $substr_len <= $len) {
                    substr ($id[1], $offset - 1, $substr_len) = ('B' x
+ $substr_len);
                    say "$tag: id[1]\n\tModified line: |$id[1]|\n";
                } else {
                    say "At Ln55: \$end: $end  > \$len: $len";
                    say "Replacements attempted beyond end of string."
+;
            }
        }
    }
}


=head OUTPUT

C:\>1091171.pl

        VAR1: : id[0]
        Modified line: |------AAAA------------------------------------
+----|

VAR1: : id[0]
        Modified line: |------AAAA-AAAAAAAAAAAAAAAAAAA----------------
+----|

At Ln26: $end: 60  > $len: 50
Replacements attempted beyond end of string.



                 NOW WORKING ON VAR2:

VAR2: : id[1]
        Modified line: |------BBBB------------------------------------
+-----------------------------|

VAR2: : id[1]
        Modified line: |------BBBB-BBBBBBBBBBBBBBBBBBB----------------
+-----------------------------|

VAR2: : id[1]
        Modified line: |------BBBB-BBBBBBBBBBBBBBBBBBB----BBBBBBBBBBBB
+BBBBBBBBBBBBBB---------------|


C:\>

=cut


__DATA__
7-10
12-30
35-60
[download]