You beat me to it!! I had it the other way round though as per the OPs example. The joined @a is only calculated once and then matched against the regex made from joining @b with .*?

my @a= qw (a b c d e f);
my @b= qw (a b c);

my $joined_a = join '', @a;
my $joined_b = join '.*?', @b;

print "yes!\n" if $joined_a =~ /$joined_b/;
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But I guess there are no prizes for coming second...