use 5.010;
use strict;
use warnings;

sub parse_file {
    my $fh = shift;
    local $/ = ''; # paragraph mode
    my @paragraphs;
    my $line_no = 0;
    while (my $para = <$fh>) {
        push @paragraphs, [ map parse_line($_, ++$line_no), split /\n/, $para];
    }
    return \@paragraphs;
}

sub parse_line {
    my ($_, $line_no) = @_;
    return unless /\S/;
    if (/^(.*)\s\@(\w+)/) {
        my $line = "$1";
        my $font_size = "$2";
        die "Invald \@fontsize declaration in line $line_no ($font_size is not a number)\n" if  $font_size =~ /\D/;

        return { text => $line, font_size => $font_size };
    } else {
        return { text => $_ }
    }
}

use Data::Dumper;
print Dumper parse_file(\*DATA);

__DATA__
First slide
Test line

Second line
Line with explicit font size @20