I'd suggest rewriting the regex as s/^i (.*)/makeImageTag($1)/egm -- the /m modifier allows ^ to match at the beginning of a "line".

You had ([^\n]*?)\n, and that really could have been ([^\n]*)\n, because those match the same way -- there's going to be a certain number of non-newlines before the next newline, and [^\n]*\n and [^\n]*?\n will match the same thing. Also, [^\n] can just be ., which saves a bit of typing. And you really don't need to remove the newline, do you?

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a (from-home) job
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;