Here's another variation on the same theme. In this case a new array is created from @ARGV with multiple words collapsed into one array element. The advantage is that this rebuilt version of @ARGV can be fed into Getopt::Long's GetOptionsFromArray method:

use strict;
use warnings;
use Getopt::Long qw(GetOptionsFromArray);

my @arglist = ();
for my $arg (@ARGV) {
    if ($arg =~ /^-/) {
        push(@arglist, $arg);
    } else {
        if ($arglist[$#arglist] =~ /^-/) {
            push(@arglist, $arg);
        } else {
            $arglist[$#arglist] .= ' '.$arg;
        }
    }
}

my ($opta, $optb, $optc);
GetOptionsFromArray(\@arglist, 'a=s' => \$opta,
                               'b=s' => \$optb,
                               'c=s' => \$optc);
for ($opta, $optb, $optc) {
    print $_,"\n";
}
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