In line 5 you attempt to (wrongly) dereference $var before the end of the statement, but you wind up dereferencing an anonymous hash (with keys 'one' and 'red') by a key ,'fish', that doesn't exist, giving $var the value undef. You can assign $var what you intended with:

my $var;
@{$var}{ qw( one two red blue )} = ('fish') x 4;
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Update: I thought you wanted to know why no error. Adapting slices to your real data:

my $ext = {};
for (keys %$mime) {
    my $case = { description => $_,
                 icon => $mime->{$_}{icon}, };
    my @exts = @{$mime->{$_}{ext}};
    @{$ext}{@exts} = ($case) x @exts;
}
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After Compline,
Zaxo