Need explanation

me too!

Could anyone explain what is going on in the following code.

$x = '<act aid = "s">'; $x=~s/<([^>]*)>/'<'.${($_=$1)=~s|\s*\=\s*|=|gs
+i,\$_}.'>'/egsi; print $x;
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The above code is used to remove the spaces before and after equal sign.

I guess it does something more, for in that case

s/\s+=\s+/=/;  # would suffice!
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Whatever, it is a substitution that in the substitution part executes code performing a further substitution. Hence it's probably not the cleanest way to do what that it does (but indeed a concise one). Note that the inner part does essentially what that I do above...

I could not understand what that \$_ and ${} does

\$_ takes a scalar reference of $_. ${} dereferences it for the purpose of interpolating it. perldoc perlref will explain all this to you much better than I ever could!!