while (<>) {
  unless (parse_line($_)) {
    my $line = <>;
    print $line;
  }
}
[download]

"Perl makes the fun jobs fun
and the boring jobs bearable" - me

I'm uncertain what your sentence here means:

The files is always in this formate where the line i want is right after the line couldn't parse

but as to a general method to use, consider this:

use strict;

open(FILE,"myfile") or die "couldn't open input file";

open(OUTFILE,">myotherfile") or die "couldn't open output file";

my @file_lines = <FILE>;

for( @file_lines )
{
  print OUTFILE $_ if /matches_this_pattern/;
}
[download]

grep -i -A 1 'couldn\'t parse' FILE.TXT | tail -1
[download]

This does what you want

tachyon

# I use the DATA filehandle here
# you will need to do an
# open(DATA,"<file.txt") || die "Oops $!\n";
# to open your file for reading

use strict;
my $line;

while (<DATA>) {
    next unless /couldn't parse/;
    $line = <DATA>;
    chomp $line;
    last;
}
close DATA;

if ($line) {
    print "found line: $line\n";
    save($line);
} else {
    print "no line found\n";
}

sub save {
    open (FILE,">>c:/my_stuff.txt") || die "Oops $!\n";
    print FILE "$line\n";
    close FILE;
    print "Saved line\n";
}

__DATA__
blah blah blah 
blah blah blah 
couldn't parse 
line i want to print
blah blah blah 
blah blah blah
[download]

I like your use of <DATA> in a quick example script. I think I'll adopt that concept.

—John

perl -ne 'if (/couldn\047t parse/) { $_ = <>; print }' test.txt
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# read a file and set a flag (boolean) if pattern is found
# we then will print the following line and reset the flag

open(OUT,$outputfilename)||die "Could not open $outputfilename for out
+put\n$!\n";
open(FILE,$inputfilename)||die "Could not open $inputfilename for read
+ing\n$!\n";
while(<FILE>){
    if($checked){
        print OUT;
        undef $checked;
        next;
    }
    if(/pattern/){
        $checked=1;
        next;
    }
    
}
close(FILE);
close(OUT);
[download]