my ($fin, $dt, $vard );Declares scalar variables. See my and perldata.

my %anc = ();
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Declares a hash variable. See perldata.

my %cnt = ();
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See line 2.

my @myfiles =();
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Declares an array. See perldata.

my @all = ();
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See line 4.Empty line. Has no effect.

@myfiles=<A*> or die "No files to open A\* for reading. $!";
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Search for files whose names start with A. If there are any, store the names to the array, if not, stop the program and complain.Empty line.

foreach $fin (@myfiles) {
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Beginning of a loop. See perlsyn.

##my $fin="A20100222.1700-1715_GWLD1E_1.xml";
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Comment. Has no effect.

open INP, "$fin" or die "Cannot open $fin for read :$!";
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Opens a file for reading, if it's possible. See open.

$fin =~ (/^A(\d{8}).(\d{4})/);
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Matches the file name against a regular expression. See perlre and perlop.

my $dt = "$1" . ":" . "$2";
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Dot is the concatenation operator.

#print "$dt\n";
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Comment.

while (<INP>)  {
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Another loop.

    if (/^<moid>.*=(\d+)</) {
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Condition.

    $anc{$1} += 1;
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See perlop.

    }
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    elsif (/^<mt>(.+)</) {
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If the first condition was not true, try this one.

    $cnt{$1} += 1;
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    }
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    elsif (/^<\/md>$/) {
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    last;
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Exit the innermost loop.

    }
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End of the loop.

}
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See the previous line.

close (INP);
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Close the filehandle.

}
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open(OUT, ">>all_announc_cnt.txt") or die "Cannot open all_announc_cnt
+.txt file for writting :$!";
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Open a file for writing (appending).

foreach $vard (sort {$a <=> $b} (keys(%anc))) {
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See sort, keys.

print  OUT "$vard:$anc{$vard}\n";
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See print.

}
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print OUT "\n";
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foreach $vard (sort (keys(%cnt))) {
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print OUT "$vard:$cnt{$vard}\n";
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#push (@all, "$vard:$anc{$vard}");
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}
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close (OUT);
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