next won't do very much for you outside of a loop. Using tools such as map with an implicit loop do not allow the use of next. However you can combine map and grep to achieve what you want:

use strict;
use warnings;

my @fields = (
    'abc abc1 abc2',
    'abc abc1 abc3 abc5',
    'abc2 abc3 abc5 abc7 abc9 abc10',
    'abc1 abc2 abc3 abc4 abc5 abc6',
    );

print grep {length > 15} map {s/\s+//g; "$_\n"} @fields;
[download]

Prints:

abcabc1abc3abc5
abc2abc3abc5abc7abc9abc10
abc1abc2abc3abc4abc5abc6
[download]

with the further advantage that you don't have to maintain the same piece of code in multiple places.

Is this suppose to be print ONLY if length of it is greater than 15 ? Currently, it's not printing anything..
What I want is after going through some sort of code,
it will take out all the \s+ and put them together
abc abc1 abc2 abc3
as

abcabc1abc2abc3

and then see if it's length is greater than 15
and print ONLY if length is greater than 15, if not, discard


print grep {length > 15} map {s/\s+//g; "$_\n"} @fields;
[download]

Here is my entire code

#!/usr/bin/perl -w

use strict;

my $ncb = $/;
my $cbn = $";

$/ = "\n\n";
$" = "\n";

my $yahoo = shift @ARGV;

open FH, "$yahoo" or die "can't open $!";

my $callid;
my $sipm;
my %data1 = ();
my $count = 0;
my @fields1;

while (<FH>) {
        chomp;
        if (m/###/) {
                next;
        }
       unless (m{(?:^\bSIP\/2\.0 \b)?(\d\d\d|^[A-Z]{3,6} ).*(Call-ID: 
+[\S]{25,80})[^ ]+: .*}s) {
               $count++;
                next;
       }    
       ($sipm,$callid) = ($1,$2);
        push (@{$data1{$callid}}, $sipm);
}
close FH;

$/ = $ncb;
$" = $cbn;

print "There were $count which could not detect\n";

#foreach $callid (sort keys %data1) {
#     my @fields = @{$data1{$callid}};
#     foreach (@fields) {
#         print map {
#            ( my $x = $_ ) =~ s/\s+//g;
#            length( $x ) < 15 ? () : $x;
#         } @{ $data1{ $callid } };
#     }
#}

foreach $callid (sort keys %data1) {
      my @fields = @{$data1{$callid}};
      print grep {length > 15} map {s/\s+//g; "$_\n"} @fields;
}
[download]

If you are not seeing the data you expect being printed then the data is probably no as you expect. Try using Data::Dump::Streamer to examine the data structure that you actually have.

---

DWIM is Perl's answer to Gödel

print map {
    ( my $x = $_ ) =~ s/\s+//g;
    length( $x ) < 15 ? () : $x;
    } @{ $data1{ $datafile } };
[download]

Having said that, length expects a scalar, so your map is evaluated in scalar context, returning the number of elements it processed. It is that number that is used as the argument to length. So, unless @fields contains 10 ** 15 elements, it's length will be less than 15 and next will be executed.

<clpm>
Please, for your next question, give us a small, complete program that we can run, rather than a snippit of something bigger, especially at this time on Saturday night ;)
</clpm>