Here you read in the whole file:

open(FILE1, $ARGV[0]) || die "Error: $!\n";
@lines = <FILE1>;
[download]

... and here you try to read some more, which fails:

while ($line=<FILE1>){
[download]

Often it helps to add debug print statements to show you the progress of your program, so you see which loops it enters and which it skips.

as the wise Corion already said in list context (given by the array @lines = < ) the diamond operator read all the file and so you r next call, in scalar context this time $line=<FILE1> , to the diamond operator will retrieve nothing.

I can just add to always use use strict; use warnings at the very beginning of your program.

Also I suggest you to always use the 3 args form for open and using a lexical scoped filehadle instead of the bareword one:

open my $file_handle, "<", $filepath or die "unable to open [$filepath] in reading monde!";

also the autodie can be useful for such open situation.

L*

PS: it seems i'm slow answering this morning..

open(FILE1, $ARGV[0]) || die "Error: $!\n";
my $save=0; 
my $saveto=''; 
while (my $line=<FILE1>){
  chomp $line; 
  my @parts=split(' ',$line,2); 
  if ( (!$save) && $parts[0] eq 'X1') { $saveto=$parts[1]; $save=1; } 
  elsif ($parts[0] eq 'X2' 
      ||($parts[0] eq 'BEGIN' && $parts[2] eq 'TAG' ) ){ 
       print $saveto."\n" if ($saveto) ; 
       $saveto=''; 
       $save=0; 
       } 
  elsif ($save) { $saveto.=' '.$line;); } 
  }
print $saveto."\n" if ($saveto) ;
[download]

Thanks for all the help.

X1: test test test
test test test tes test test
X2: no no no no
no no no no no nono non no no nononono no no  no
X3: yes yes yes
hi hi hi hi hi hi hi hi hi hi hi hi
[download]

#!/usr/bin/perl
use strict; 
use warnings;
open (FILE, '<', $ARGV[0]) or die "Could not open file: $!";
my $saveX1=0;
my $saveX2=0;
my $saveX3=0; 
my $savetoX1=''; 
my $savetoX2='';
my $savetoX3='';
while (my $line=<FILE>){
  chomp $line; 
  my @parts=split(' ',$line,2); 
  if ( (!$saveX1) && $parts[0] eq 'X1') { $savetoX1=$parts[1]; $saveX1
+=1; } 
  elsif ($parts[0] eq 'X2' ||($parts[0] eq 'BEGIN' && $parts[2] eq 'TA
+G' ) ){ 
       #print $savetoX1."\n" if ($savetoX1) ; 
       $savetoX1=''; 
       $saveX1=0; 
       } 
  elsif ($saveX1) { $savetoX1.=' '.$line; } 

  
  if ( (!$saveX2) && $parts[0] eq 'X2') { $savetoX2=$parts[1]; $saveX2
+=1; } 
  elsif ($parts[0] eq 'X3' ||($parts[0] eq 'BEGIN' && $parts[2] eq 'TA
+G' ) ){ 
       #print $savetoX2."\n" if ($savetoX2) ; 
       $savetoX2=''; 
       $saveX2=0; 
       } 
  elsif ($saveX2) { $savetoX2.=' '.$line; } 

  
  if ( (!$saveX3) && $parts[0] eq 'X3') { $savetoX3=$parts[1]; $saveX3
+=1; } 
  elsif ($parts[0] eq 'BEGIN' && $parts[2] eq 'TAG' ){ 
        #print $savetoX3."\n" if ($savetoX3) ; 
        $savetoX3=''; 
        $saveX3=0; 
       } 
  elsif ($saveX3) { $savetoX3.=' '.$line; } 
  
  
  
  
  }
print "X1: ".$savetoX1."\n" if ($savetoX1) ; 
print "X2: ".$savetoX2."\n" if ($savetoX2) ; 
print "X3: ".$savetoX3."\n" if ($savetoX3) ;
[download]

I'm not entirely sure I understand the rules by which you want newlines to be added to the output or not. In the following, if you don't want newlines in the output, change the two occurrences of "\n" to " ".

#!/usr/bin/env perl
use warnings;
use strict;

my %data;
my $cur_tag;
while (<DATA>) {
    chomp;
    if ( my ($tag,$line) = /^(X\S+)\s+(.+)$/ ) {
        $data{$tag} .= "\n" if exists $data{$tag};
        $data{$tag} .= $line;
        $cur_tag = $tag;
    }
    elsif ($cur_tag && !/^BEGIN_TAG/) {
        $data{$cur_tag} .= "\n".$_;
    }
}

for my $tag (sort keys %data) {
    print "$tag: $data{$tag}\n";
}

__DATA__
BEGIN_TAG
X1 test1 test1 test1
X2 no1 no1 no1 no1
X3 yes1 yes1 yes1
BEGIN_TAG
X1 test2 test2 test2
test3 test3 test3
X2 no2 no2 no2 no2 no2
no3 no3 no no3 no3
no4 no4 no4 no4 no4 no4 no4
X3 hi2 hi2 hi2 hi2 hi2 hi2
hi3 hi3 hi3 hi3
hi4 hi4
[download]

Output:

X1: test1 test1 test1
test2 test2 test2
test3 test3 test3
X2: no1 no1 no1 no1
no2 no2 no2 no2 no2
no3 no3 no no3 no3
no4 no4 no4 no4 no4 no4 no4
X3: yes1 yes1 yes1
hi2 hi2 hi2 hi2 hi2 hi2
hi3 hi3 hi3 hi3
hi4 hi4
[download]

Update: Inverted "\n" vs " ".

Wow that's it thank you!!!!

Sorry, i missed the underscore in BEGIN_TAG, and i even thought i remembered going up to look to make sure. That meant that BEGIN_TAG didnt serve as the ending trigger like it should have, instead i was looking for "BEGIN TAG" with a space. So this would fix my example.

open(FILE1, $ARGV[0]) || die "Error: $!\n";
my $save=0; 
my $saveto=''; 
while (my $line=<FILE1>){
  chomp $line; 
  my @parts=split(' ',$line,2); 
  if ( (!$save) && $parts[0] eq 'X1') { $saveto=$parts[1]; $save=1; } 
  elsif ($parts[0] eq 'X2' 
      ||($parts[0] eq 'BEGIN_TAG' ) ){ 
       print $saveto."\n" if ($saveto) ; 
       $saveto=''; 
       $save=0; 
       } 
  elsif ($save) { $saveto.=' '.$line;); } 
  }
print $saveto."\n" if ($saveto) ;
[download]

And you never said you wanted to capture all of them to the end, so after each set was complete (bu hitting X2 or (now fixed to ) BEGIN_TAG i printed it, AND reset $saveto to blank. When you commented out the print, you didnt comment out the reset to blank, so only the last set was saved.

Please show wrong and expected output and code with proper indentation.

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!}

Edited with wrong and expected output plus formatted source