Thanks! The reply by Khen1950fx led me to conclude that this works - the only difference being the presence of round brackets... 
#!/usr/bin/perl
use PDL; 
use PDL::GSLSF::GAMMA;
my ($out) = gsl_sf_gamma(0.4);
print "$out\n";

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In reply to Re: Using PDL::GSLSF::GAMMA by Anonymous Monk
in thread Using PDL::GSLSF::GAMMA by wannymahoots

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