Quick, untested Algorithm::Loops hack, but it doesn't require a huge list of nested arrays to be stored before you sort through them looking for the ones to keep:

use Algorithm::Loops qw( NestedLoops );

sub GenLances
{
    my( $hvMechs, $members, $maxWeight )= @_;
    my @mechs= keys %$hvMechs;
    my $iter= NestedLoops(
        [ [0..$#mechs], (sub{[$_+1..$#mechs]})x($members-1) ],
        {
            OnlyWhen => sub {
                return   if  @_ != $members;
                $weight= 0;
                $weight += $_->{weight}  for  @$hvMechs{@mechs[@_]};
                return $weight <= $maxWeight;
            },
        }
    );
    return sub {
        my @idx= $iter->();
        return @mechs[ @idx ];
    };
}

# ...

my $iter= GenLances( \%mechs, 3, 150 );
my @lance;
while(  @lance= $iter->()  ) {
    # ...
}
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