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
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