I'm totally at a loss,

In the code below, I am getting input from a form.
This input contains the string <insert>.
I then open a directory with several text files.
The text files contain 0ne line of text each.
I then put the place the contents of each text file
into the variable $line.

I want this to do a simple thing.
Insert the value of $line into the right
place in $input, and print the results.

The code below prints the value of $input, but
leaves the value of $line blank.
If I put "print "$line";" above "print "$input";"
the value of $line is printed, so I know that $line
has a value.

question is...why won't the regex do the substitution?
I can change the regex to

$input =~ s/<insert>/sometext/;

and it faithfully substitutes sometext for <insert>.
I know you can use a variable in a regex, and I'm totally
stumped. I know it's probably something simple, just a little
out of my experience.

my $input = $form{'input'};
opendir THEDIR, "$config{'path'}" or &error("packs directory could not be opened.");
my @allfiles = readdir THEDIR;
closedir THEDIR;
my $file;
foreach $file (@allfiles) {
open THEFILE, "$config{'path'}$file";
my ($line) = <THEFILE>
close THEFILE;
chomp ($line);
$input =~ s/<insert>/$line/;
print "$input";
}
}

Thanks for any help

Bob