Programming

$ cat abc
abc
def
ghi
jkl

abc def
ghi jkl

$ cat add_line.pl
#!/usr/bin/perl -w

my $count=1;
#my $line=undef;
my @mem_line;
my $i=0;
my $x=0;

while(<>)
{
chomp;
$x=$count % 2;
#print "$x $_\n";
        if ( $x == 1 )
        {
                $mem_line[$i] = "$_";
        }
        else
        {
                $mem_line[$i - 1] .= " $_\n";
        }
$count+=1;
$i+=1;
}

print "@mem_line";

./add_line.pl abc
Use of uninitialized value in join or string at ./add_line.pl line 26, <> line 4.
abc def
ghi jkl

while(<>)
{
chomp;
$x=$count % 2;
#print "$x $_\n";
        if ( $x == 1 ) 
        {   
                $mem_line[$i++] = "$_";
        }   
        else
        {   
                $mem_line[$i - 1] .= " $_\n";
        }   
$count+=1;
#$i+=1;
}

awk '{getline l;print $0,l}' input