Well, you could use a useless assignment to (). Except it's not useless. As you've already demonstrated, a list assignment in scalar context returns the number of scalars returned by its RHS.

( my @a = `route print`=~ m!0.0.0.0!g ) > 3
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( () = `route print`=~ m!0.0.0.0!g ) > 3
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my $n = $str =~ m/this/g;
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my $n = () = $str =~ m/this/g;
my $n = grep 1, $str =~ m/this/g;
my $n = map $_, $str =~ m/this/g;
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my $n = 0; ++$n while $str =~ m/this/g;
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