There are two basic ways to insert text into a string. The substr function is preferred if you know the offset where the new text is to go,

substr $_, $offset, 0, $added_text;
# or
substr( $_, $offset, 0) = <<EOT;
Some text
to add.
EOT
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The other way is with the substitution operator, s///. That is preferred when the insertion position is determined by context, i.e. by regex matching. To match across line ends, you may want to use the /s flag so that '.' is allowed to match "\n", and it sounds as if you want to do multiple matches, so the /g flag is also called for. s/$pattern/$new_text/sg; More detail about your pattern and the substitute text would allow better advice.