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]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.