If I were going to do this and wanted to have it expandable, I'd probably plan to make a hash like this (you can simplify it if you don't care about keeping track of student IDs and never have students with the same names): 
my %records = (
    12445 => {
        name => 'Jack',
        scores => [ 45, 10 ],
    },
    234254 => {
        name => 'Jill',
        scores => [ 45, 10 ],
    },
);

[download]

If the data are consistently in the format shown, the following will make the hash (it can accept scores w/ decimals). It then calculates and reports the mean score.

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use List::Util 'sum';

my %records;

while ( my $name = <DATA> ) {
    my ($id)    = <DATA> =~ /(\d+)$/;
    my ($score) = <DATA> =~ /(\d+\.?\d*)$/;
    <DATA>;
    chomp( $name, $id, $score );

    $records{$id}{name} = $name;
    push @{ $records{$id}{scores} }, $score;
}

for ( sort { $a <=> $b } keys %records ) {
    my $name   = $records{$_}{name};
    my @scores = @{ $records{$_}{scores} };
    my $mean   = sum(@scores) / @scores;
    say "$name ($_) has an average score of $mean";
}

__DATA__
Jack
Student ID - 12445
Math Score - 45

Jill
Student ID - 234254
Math Score - 90

Jack
Student ID -12445
Math Score2 - 33

Jill
Student ID - 234254
Math Score2 - 10

[download]

OUTPUT: 

Jack (12445) has an average score of 39
Jill (234254) has an average score of 50

[download]

In reply to Re: Selecting successive lines by frozenwithjoy
in thread Selecting successive lines by zee3b

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