> Binary "=~" binds a * scalar expression * to a * pattern match * .

perlop also says

If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time.

this means there is a DWIM behaviour to fall back to match m// and the following are equivalent:

  DB<100> "abc" =~ m/a/
 => 1

  DB<101> "abc" =~ "a"
 => 1

  DB<102> "abc" =~ qr(a)
 => 1
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please note that you could also use a plain string (line 101), but still without /g.

ِAGAIN /g transforms m// and s/// to different commands with different contextual behaviour!

for instance

The "/g" modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern.

In scalar context, each execution of "m//g" finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the "pos()" function; see "pos" in perlfunc. A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the "/c" modifier (e.g. "m//gc"). Modifying the target string also resets the search position.