Have a look at the following:
File file.dat:

"I000001","dummy","dummy"
"I000003","dummy","dummy"
"I000002","dummy","dummy"
"I000004","dummy","dummy"
"I000010","dummy","dummy"
"I000019","dummy","tommy"
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Perl script:

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;
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Besides, Limbic~Region's proposal to use the magic of the increment operator is interesting too, try

perl -e "$x = 'I000199'; print ++$x"
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to get acquainted with it.

Hope this helped.
CombatSquirrel.

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Entropy is the tendency of everything going to hell.