my %lines = map { ($_, 1) } qw/1 3 5 7/;  
open(IN, "<grep") or die;
while (<IN>) {
   print OUT $_ if $lines{$.};
}
[download]

open (IN, "<grep");
my %lines;
while (<IN>){
  $lines{$_} = 1;
}
close IN;

open (OUT, ">gout");
# assumes input data comes from stdin
while (<>){
  print OUT $_ if exists $lines{$.};
}
close OUT;
[download]

I don't use perl for things for which there are standard unix utils.

$ cat text
first
second
third
fourth
fifth
sixth
seventh
eighth
$ cat nums
1
4
6
7
$ ed -s text < nums
first
fourth
sixth
seventh
[download]

Update: this sucks if the text file is too large to load in memory. In that case,

Update: here's a perl solution too:

perl -we 'open N, "<", shift or die; $p = 0; while(defined($n = <N>)) 
+{ $s = <> for $p + 1 .. $n + 0; print $s; $p = $n; }' nums text
[download]

Update: if anyoune knows how to convert the above snippet so that it uses either $. or IO::Handle::input_line_number, please reply me, cause I can't figure out how to do that.

#! /usr/bin/perl -w

use strict;
use Fcntl 'O_RDONLY';
use Tie::File;


# Path to file containing lines as arg
my $linesPath = shift;

# Tie the file read-only, if you want to read only, to be safe
tie my @linesFile, 'Tie::File', $linesPath, mode => O_RDONLY
    or die "Can't open file $linesPath: $!";

while(<DATA>) {
    chomp;

    # $_ - 1, I'm assuming that the list isn't 0-based
    print "$linesFile[$_ - 1]\n";
}

untie @linesFile;

__DATA__
1
3
4
7
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the line numbers to print are stored in a file.

1
3
5
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text file

once upon a time
...
big bear
....
shot dead
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so the lines i want to print are 1 3 and 5, to a separate output file thanks

open (OUT, ">gout");

while (<>){
chomp;
$line = $_;
push @array, $line;
}

print @array;
open (IN, "<grep");

while (<IN>){

next unless $. == $array[$i++];
print OUT $_;
#if ($. == @array) {print OUT $_}
}
[download]

This code has a well-hidden bug (or maybe I'm too tired): you always increment $i even if the equalty doesn't stand. Change

next unless $. == $array[$i++];
[download]

to

next unless $. == $array[$i];
$i++;
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and it should work.

Also, you should really add or die "open: $! to your open statement.

Update: and to eliminate a warning, you may want to replace

next unless $. == $array[$i++];
print OUT $_;
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to

next unless $. == $array[$i];
print $_;
++$i < @array or last;
[download]

Anonymous Monk gives some code he's started so you are a bit unfair I think.

As someone who creates exercises for a living, this is exactly the kind of exercise I would create. I also note the absence of "real world" indicators in the question, again leading toward an abstract problem that someone would not have occur "in the wild".

No, I'll stand by my assessment. Homework, with some of the work done, but missing some essential piece. Nothing that should be answered, because the student should go back to the teacher for answers, not rely on the virtual brain of the monastery.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

use strict;

my ($handle, @lines, $c);

open($handle, "lines");
@lines = <$handle>;
close($handle);

chomp @lines;
@lines = sort {$a <=> $b} @lines;

open($handle, "text");
while ($c <= $#lines) {
    <$handle> for (($.)..($lines[$c++]-2));
    print $_ = <$handle>;
}
close($handle);
[download]

Just for fun.....

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

my @prints;
while (<DATA>) {
    chomp;
    $prints[--$_]="print this one";
}

open IN,  '<', 'grep' or die 'dulce et decorum est';
open OUT, '>', 'gout' or die 'dulce et decorum est';
while (<>) {
    print OUT if shift @prints
}
close IN;
close OUT;

__DATA__
1
3
5
8
[download]

Cheers,
R.