use strict;
use warnings;

#  I assume the actual code will obtain this list using
#  glob or similar
my @allfiles = qw /
baz.txt
baz.epub
baz.doc
baz.pdf
bar.epub
boo.epub
boo.txt
/;

my %by_ext;
#  %by_fbase is actually redundant below,
#  but is maybe useful for other things
#  so I have left it in
my %by_fbase;  

foreach my $file (@allfiles) {
    #  should use a proper filename parser here
    #  like File::Basename, but a split will serve
    #  for the purposes of an example.
    my ($name, $ext) = split /\./, $file;
    
    $by_ext{$ext}{$name} = $file;
    $by_fbase{$name}{$ext} = $file;
}

foreach my $name (keys %by_fbase) {
    #print "$name\n";
    no autovivification;
    #  could use exists in this check if you want to avoid autoviv,
    #  but file names should evaluate to true if they have
    #  an extension, even if the name part evaluates to false
    if ($by_fbase{$name}{pdf} && $by_fbase{$name}{epub}) {
        #  do stuff
        print "$name has epub and pdf extensions: "
          . "$by_fbase{$name}{epub} $by_fbase{$name}{pdf}\n";
        #  now do stuff like moving files since you can iterate over
        #  the values of the relevant subhash
        foreach my $file (values %{$by_fbase{$name}}) {
            print "now do something to $file\n";
        }
        
    }
}