use strict;
use warnings;
open my $in, '<', 'infile';
open my $out, '>', 'outfile';
my @values;
while (my $line = <$in>) {
    chomp $line;
    my ($value,) = $line =~ /^line\d+=(.+)/;
    push @values, $value if (defined $value);
}
print $out join('/', @values);
close $in;
close $out;