If you're working in your target directory, the error message you've asked about appears because you're trying to open the parent directory (..), the current directory (.) and subdirectories, if any, as if they were files, because opendir captured those to your array of files.

Knock those out of your @files before trying to open anything. (see http://perldoc.perl.org/perlfunc.html)

You have some other problems in this script. Pay special attention to graff's discussion of path. Some will be easily solved if you add use diagnostics; to your pragmata; some are in nature of your failure to test the open at line 21 (and -- in the same line -- failure to use what's now considered best practice: 3 arg open with lexical filehandles.)

Perhaps most critical among the problems you didn't ask about is the attempt at line 23 to test an array (in scalar context) for a match -- and you need to read about qr/.../ -- either qr or in Quote and Quote-like Operators to make your regex match what you expect... and at line 24, where you'll find you're pushing something quite unexpected onto @found.

Updated for grammar, markup and clarity

Update 2 (Warning: Sunday morning content): This is one way of attacking your target (and problem) that's along the lines you initially tried:

#!/usr/bin/perl
use warnings;
use strict;
# use diagnostics;
#regexp.pl   # 911425

print "Enter the full path to the directory you want to search: ";
my $folder = <>;
chomp $folder;
chdir($folder);

opendir(DIR, $folder) or die "Can't open $folder, $!";
my @files = readdir(DIR) or die "Can't readdir $folder, $!";

print "What's the phrase you're looking for?: ";
my $find = <STDIN>;
chomp $find;
my $searchterm = qr/$find/;

my (%found, $found, $file);

for $file(@files) {
    next if ($file =~ /^\./);
    next unless (-T $file);       # text files only (excludes binary f
+iles such as *.doc or .xls)
    open(my $fh, '<', $file) or die "Can't open $file: $!";
    my @content = <$fh>;
    for my $line(@content) {
        if ($line =~ /$searchterm/i) {
            my $key = $file;
            $found{$key} += 1;
          }
    }
}
 
while (my ($key, $value) = each %found) {
        print "$key has \t $value instance(s) of \t \"$find\"\n";
}
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