It looks like it behaves like a regex delimiter without the m.

perl -MO=Deparse -e '$a = "foo"; print 1 if $a =~ "foo"'
$a = 'foo';
print 1 if $a =~ /foo/;
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It will match a pattern, too, not just a literal string. print 1 if $a =~ "fo."; works fine.

Neat.