I'm trying store each word from a txt file as an element in an array, join the elements using a dash, and print the joined string so I can check whether everything was split in the right places (so if there's a dash where I wanted the file to be split, then I know things worked properly)

Displaying with a dash for verification is hardly ideal - what if you want to allow hyphenated words, say? I also think it's more flexible to read from a test file (passed as the first argument to the program) rather than hard-wiring STDIN. You might further like to consider how to write an automated test for this.

In case it is of use, this is how I'd go about it:

use strict;
use warnings;
use Data::Dumper;

my $fname = shift or die "usage: $0 fname\n";
open(my $fh, '<', $fname) or die "error: open '$fname': $!";

# Slurp file contents into string $contents
my $contents = do { local $/; <$fh> };
close $fh;

# Extract what you want, for example
my @words = $contents =~ /\w+/g;

# ... or split on what you don't want, for example
# my @words = split /\s+/, $contents;

# In both approaches above you can tweak the regex to suit

# Print out the extracted word list to verify
for my $word (@words) {
   print "word='$word'\n";
}

# ... or use Data::Dumper
print Dumper( \@words );

# ... or write an automated test with specified input and expected out
+put
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