use strict;
use warnings;
use Data::Dumper;
my @data = ...;
my @sections;

for (@data) {
    for my $i (1 .. 3) {
        my $j = $i+1;
        if (m/Sec_$i/ .. m/Sec_$j/) {
            # füge zeile in das array von $sections[$i-1] ein
            push @{ $sections[$i-1] }, $_;
        }
    }
}
print Dumper \@sections;