"I000001","dummy","dummy"
"I000003","dummy","dummy"
"I000002","dummy","dummy"
"I000004","dummy","dummy"
"I000010","dummy","dummy"
"I000019","dummy","tommy"

##</code><code>##

my $content;
open FILE, '+<', 'file.dat' or die "Could not open 'file.dat': $!\n";
$content = $_ for (<FILE>);
$content =~ s/(?<=^"\w)(\d{6})/sprintf "%06d", $1 + 1/e;
print FILE "\n$content";
close FILE;

##</code><code>##

perl -e "$x = 'I000199'; print ++$x"