$ perl -Mstrict -Mwarnings -E '
open my $wordsFH, q{<}, \ <<EOF or die $!;
cat
dog
EOF

my %words =
   map { chomp; $_ => 0 }
   <$wordsFH>;

my $rxWords = do {
   local $" = q{ | };
   qr{(?x) \b ( @{ [ keys %words ] } ) \b };
   };
say qq{Regex is $rxWords};

open my $textFH, q{<}, \ <<EOF or die $!;
The cat scattered doggerel words over the
poor dog as it doggedly ignored the catastrophe
the cat was causing
EOF

my $text = do {
   local $/;
   <$textFH>;
   };

$words{ $1 } ++ while $text =~ m{$rxWords}g;

say qq{$_ => $words{ $_ }} for sort keys %words;'
Regex is (?^u:(?x) \b ( dog | cat ) \b )
cat => 2
dog => 1
$