Quote

If the PATTERN evaluates to the empty string, the last success-
fully matched regular expression is used instead. In this case,
only the "g" and "c" flags on the empty pattern is honoured - the
other flags are taken from the original pattern. If no match has
previously succeeded, this will (silently) act instead as a gen-
uine empty pattern (which will always match). perlop#Regexp_Quote_Like_Operators

1
2
3
4
5
6

$\="\n";
print "b" =~ //;        #1
print "a" =~ /a/;       #1
print "a" =~ /b/;
print "b" =~ //;
print "a" =~ //;        #1