Do you want to parse the complete file into a nested data structure? There are plenty of modules on CPAN. Not sure which one is the best here (Parse::RecDescent is nice) Or is it just a pattern matching problem:

my $filecontent = "{type line} {series {{1 {{x_data {1 2 3 4 5}} {y";

my ( $xdata ) = $filecontent =~ / x_data .*? ({.*?})/xms;
my @x;
while ($xdata =~ /(\d+)/g) {
   push @x, $1;
}
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?