my $content = do { local $/; <DATA> };

my ( $ln, $start) = ( 1, 0);

while ( $start < length $content ) {
    if ( substr( $content, $start) =~ s/.../.../ ) {
        print "line $ln changed\n";
    }
    $start = 1 + index( $content, "\n", $start);
    ++ $ln;
}
__DATA__
aaa
bbb
ccc
ddd
eee