Just an outlined hint, but you could use recursion and Perl's ability to store the returned list in separate variables.

use strict;
use warnings;

sub beaver_find
{
    my ($cond, @l) = @_;

    return () unless @l;

    my $head = shift @l;
    return $cond->( $head ) 
                ? ($head, @l)
                : beaver_find( $cond, @l );
}

my $cond = sub { my $x = shift; return $x > 0 };

my @l = ( -1, -3, -2, 10, -1, -3, -2 );

my( $head, @tail ) = beaver_find( $cond, @l );

print qq|
    Head: $head
    Tail: @tail
|;
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