my $left_text   = [ qw{ when rome romans} ];
my $right_text  = 'When in Rome, do as the Romans.';
my $regex = join '|', map quotemeta, @$left_text;
   $regex = qr{\b(?:$regex)\b}i;
my $found  = () = $right_text =~ m/$regex/g;

print $found, $/;
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The  $found = () = trick first sets the RHS in list context, and then by obtains the number of elements in the list.