Try using  <code> </code> blocks to show your file contents.

As to the question, i would try using regular expressions in multiline mode and replacing the newline if it's followed by whitespace with a single space character.

Update:

#!/usr/bin/perl -w
use strict;

$/ = undef;
my $filecontents = <>;

$filecontents =~ s/\n\s+//gm;
print $filecontents;
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Regards, Luke

Re: Appending lines starting with white space to the previous line reads the entire file into memory. This version processes the file line by line thanks to redo and a one line lookahead.

chomp( my $line = <$fh> );
while (defined($line)) {
   chomp( my $next = <$fh> );
   if (defined($next) && $next =~ s/^\s+//) {
      $line .= $next;
      redo;
   }

   ... do something with $line ...

   $line = $next;
}
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Update: Tested. Small syntax fix.

my $i=0;
my $temp = "ARR";
while (<FILE>)
{
    if ($_ !~ /^\s+/) {
        chomp;
        $i++;
        @{$temp{$i}} = ($_);
    }
    else {
        my $temp2 = $_;
        chomp ($temp2);
        $temp2 =~ s/^\s+//g;
        push (@{$temp{$i}},$temp2);
    }
}
close (FILE);
    
for $j(0..$i)
{
    print "@{$temp{$j}}"."\n";
}
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Hi,

this code parses the input linewise and prints directly to STDOUT, so you can redirect the result to another file.

while ( my $line = <DATA> ) {
    chomp $line;

    # remove the spaces from start of line
    if ( $line =~ s/^\s+// ) {
        # print line if successful replacement
        print $line;
    }
    else {
        # print newline (not for first line)
        print "\n" if $. != 1;
        print $line;
    }
}

__DATA__
first
    second
    third
first
    second
    third
first
    second
first
[download]

Update:

shorter version:

while ( my $line = <DATA> ) {
    chomp $line;

    # remove the spaces from start of line
    if ( ! ( $line =~ s/^\s+// ) ) {
        # print newline (not for first line)
        print "\n" if $. != 1;
    }

    print $line;
}
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while(($_ = scalar <>))  {
# remove the new line from that line.
  chomp;
  if ( /^\s+/ )  {
        # if there is no previous line then print the warning and cont
+inue reading the next line.,
        if ( not defined $cur_line )  {
              warn "continuation of no previous line\n";
              next;
        }
  # since it is a continuation line, concatenate it with the previous 
+line.,
  $cur_line.=$_;
  } else {
        # if the line does not contains the tab, then take that line a
+s the current line.,
        if (defined $cur_line) {
            # print the concatenated line.
            print $cur_line . "\n";
        }
        # store the current line in the variable
        $cur_line = $_;
    }
}
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$_ = <>; chomp; print;
while (<>) {
    chomp;
    print "\n" unless s/^\s+//;
    print;
}
print "\n";
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( my $line = ( join "", <>  ) ) =~ s/\n\s+//g;    
print $line;



______DATA_______

firstline54324563213612531436436463246324634
         secondlidfndsfdsfdsfd
         thirdlinedsfsdf
firstline54324563213612531436436463246324634
         secondlidfndsfdsfdsfd
         thirdlinedsfsdf
         fourthlinedsfsdf
         fifthlinedsfsdf

_____OUTPUT______

firstline54324563213612531436436463246324634secondlidfndsfdsfdsfdthird
+linedsfsdf
firstline54324563213612531436436463246324634secondlidfndsfdsfdsfdthird
+linedsfsdffourthlinedsfsdffifthlinedsfsdf
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open(FILE,"f1") or die $!;
while(<FILE>){if($_!~/^\s+/){push(@array1,'>>'.$_);}else{push(@array1,
+$_);}}
$array=join('',@array1);
$array=~s/\s+//g;
$array=~s/>>/\n/g;
print $array;
[download]