use strict;
use Data::Dumper;

open(FILE, "<", "file.txt");
my $active_file;
my %files_with_bad_media;
while (<FILE>) {
    if (m/^checking image file <(.*?)>/) {
        $active_file = $1;
    } elsif (m/checking files file/) {
        $active_file = undef;
    } else {
        if (m/media does not exist/) {
            if (defined($active_file)) {
                $files_with_bad_media{$active_file} = 1;
            }
        }
    }
}
close(FILE);
print Dumper(\%files_with_bad_media);