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use strict;
use warnings;

...
      push @out_com, "$comm  -mdb $_" for @db;
 }
 print join "\n", @out_com;

$ perl commands.pl
HPRMCLI exch create -s EXCHDAGNODE1   -mdb db1
HPRMCLI exch create -s EXCHDAGNODE1   -mdb db2
...
HPRMCLI exch unmount   -mdb db2
HPRMCLI exch remove   -mdb db1
HPRMCLI exch remove   -mdb db2

for my $database (@db) {
      push @out_com, "$_  -mdb $database" for @commands;
}

$ perl commands.pl
HPRMCLI exch create -s EXCHDAGNODE1   -mdb db1
...
HPRMCLI exch validate  -level Full  -mdb db2
HPRMCLI exch unmount   -mdb db2
HPRMCLI exch remove   -mdb db2

for my $comm (@commands) {
     print "$comm  -mdb $_ \n" for @db;
 }