Wow.

You are matching (first part of the s///egsi):

<([^>]*)>
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, which means "the content of every tag", i.e. the act aid = "s". Due to the round parentheses, what you match is put inside $1. Moreover, due to the gsi modifiers, the substitution will be applied to the whole string (g), without caring of case (i) and trating newlines as any other character (s).

The interesting part comes with the "e" modifier. This tells Perl that the "replacement" part of the substitution is an expression, not a bunch of characters. The espression is the following:

'<' . ${($_=$1)=~s|\s*\=\s*|=|gsi,\$_} . '>'
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that is the concatenation of three parts: the enclosing angle brackets and the "content". When you use the ${ EXPRESSION } construct, you are de-referencing a reference to a scalar, so:

• you have to take care that EXPRESSION evaluates to a reference to a scalar, and
• you can do whatever you want before this "return value"

EXPRESSION is:

($_ = $1) =~ s|\s*\=\s*|=|gsi , \$_
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that is a canned sequence of sub-expressions. The last is what "returned", and you can notice that it is a reference to a scalar (i.e. a reference to \$_), which is what we want (see first bullet). The first sub-expression is an assignment followed by yet another substitution, using the | character as a separator.

The assignment is necessary because $1 is read-only. So you use the $_ variable as a temporary copy, that you can modify via the substitution.

The substitution can be rewritten as:

s{\s*\=\s*}
 {=}gsi
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which means that any "sequence of zero or more spaces, followed by an equal sign, followed by a sequence of zero or more spaces" is replaced by "an equal sign". Note, again, the use of the g modifier (which applies the substitution to the entire string) and of the s modifier ("treat newline as any other char"). The i modifier is not needed in this case, but it doesn't hurt. Why do the trick <c>${ ... , \$_ }, then? Because it lets you embed code to define the "contents" part without the need to have separate statements (I guess).