I recently came across this same trouble, and found a quick and easy solution that I thought I'd share. When you compiled the full PGPLOT distribution, you likely compiled it by something along the lines of (check the pgplot docs) 
makemake "wherever the source files are" linux g77_gcc

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but looking at the top line of your output of PGPLOT-2.20, it says 
Found compiler g77
Found compiler gfortran
ExtUtils::F77: Using system=Linux compiler=GFortran
...
ExtUtils::F77: Compiler: gfortran

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and therein lies the rub: pgplot was compiled and linked against G77, and PGPLOT-2.20 is compiling/linking against gfortran and the two are not compatible. Take a look at the Makefile.PL file in the PGPLOT-2.20 directory and UNcomment the line that reads something like 
#use ExtUtils::F77 qw(generic g77);

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and this will force PGPLOT-2.20 to use g77 instead of gfortan like it wants to. Other suggestions on the interwebs suggest compiling pgplot itself through gfortran, but this leaves you hanging dry if you need some of the more common output types. This took a while to figure out, but it works as expected for me.

In reply to Re^3: Problems installing PGPLOT.pm by Anonymous Monk
in thread Problems installing PGPLOT.pm by nickschurch

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