%{$excel_hash}: treat the variable $excel_hash as a hash. perlreftut perlref perldsc
(keys %{$excel_hash}): return a list of keys for the given hash. keys
for (keys %{$excel_hash}) {: loop over the hash keys, setting $_ (by default because no variable was given to for) to each key in turn. For Loops
for (keys %{$excel_hash}) { print "$_\n"; }: print each hash key (note: not the value, just the key).
for (keys %{$excel_hash}) { print "key:$_ = $excel_hash->{$_}\n"; }: print keys and values.

For a complete view of the structure of $excel_hash, try:

use Data::Dumper;
print Dumper($excel_hash);
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