Untested, but this might get you close: 

#! perl -slw
use strict;

open ZCAT, '|-', 'zcat *.gz' or die;

my %uniq;

while( <ZCAT> ) {
    my( $varname ) = m[!dataset_([^=]+)=] or next;
    $uniq{ $varname } = 1;
}

close ZCAT;

print "uniq names sorted";
print for sort keys %uniq;

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
RIP an inspiration; A true Folk's Guy

In reply to Re^2: How would you do this? by BrowserUk
in thread How would you do this? by david_lyon

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