I've heard a future version of Perl (5.10.x ?) is going to have a new operator that assigns if undefined, ie.

my $foo ||= 1; # $foo is set to '1' if it was previously a false value

my $bar //= 1; # $bar is set to '1' only if it was undefined (0 and em
+pty string don't count).
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It would also be useful to have // (or equivilent) as a normal short circuit operator as well, although this is really syntactic sugar for defined and existing operators.

In the meain time, personally, I would use:

sub routine {
     my $input_ref = {@_};
     my $var1      = defined $input_ref->{var1} ? $input_ref->{var1} :
+ 1;
}
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Not quite as pretty, but better than a full if block.