It seems to work as expected and, as far as I can tell, as you wish for me:

use strict;

my $text = <<TEXT;
<p>Before new line.
After new line.</p>
TEXT

print "Before: |$text|\n";

$text=~s#<p>(.+?)<\/p>#"<p>".&enter($1)."<\/p>"#gsie;

print "After: |$text|\n";


sub enter
{
my $new;

($new)=$1;
$new=~s/\n//gsi;
return $new;
}
[download]

Prints:

Before: |<p>Before new line.
After new line.</p>
|
After: |<p>Before new line.After new line.</p>
|
[download]

If that is not what you want then perhaps you should provide your own sample and expected output?

That aside, there are a few things worth mentioning:

• it is generally considered bad practice to call subs using & these days
• you pass a parameter to the sub, but don't use it (see below)
• you use $1 as a "global" to the sub which is bad in itself, but also there is no need! (see below)
• hand parsing HTML is a bad, mad and dangerous thing to do. Use HTML::TreeBuilder or one of the other modules designed for the purpose
• (minor) you use # to delimit the parts of the substitution, but quote the / - / is not special if not the delimiter so doesn't need to be quoted

The sub would be better written:

sub enter
{
my $new = shift;

$new=~s/\n\s*/ /gi;
return $new;
}
[download]

Note the alteration to the regex so that a space is inserted in place of a new line, but that leading white space on the line is removed:

After: |<p>Before new line. After new line.</p>
|
[download]

Is it possible that you are confusing HTML paragraph tags with the newline value(s) used by your system? What you see in your browser is rendered (created, especially for you!) in response/obedience to the paragraph tags (<p> and </p>

As is, your code does nothing for me, except spew warnings and errors.

The HTML in your $text contains NO newlines. What's in the original; ie, what did your assign to $text?

Your phrase, "enter marks," coupled with your $text, and the .&enter($1) (concatenated sub call ?!??) on the replacement side the substitution leave /me confused and guessing

Update: rephrased, added (I hope) clarity to the far-too-telegraphic original.

open FILE, $filename;

while (<FILE>) {
 chomp($_); # removes \n.
 # do something to $_
}
[download]

Taking another look at your post, it seems as if what you actually want to do is remove paragraph tags from your html. In which case, apart from what others have said, you'll be wanting to look at a regex that looks more like: $new=~s/<p>|<\/p>//gsi