Further inspection of the problem leads me to the conclusion that this may not be related to encode. I fired up my Debugger and performed some tests. Notice how the Character \N{greek:alpha} was found on Linux, but not on AIX:
AIX 7.2:

  DB<1> use charnames qw(greek)

  DB<2> print "\N{alpha}"
Use of uninitialized value $txt in pattern match (m//) at /usr/opt/per
+l5/lib/5.20.1/_charnames.pm line 459.
 [...]
Unknown charname 'alpha' at (eval 9)[/usr/opt/perl5/lib/5.20.1/perl5db
+.pl:732] line 2, within string

  DB<3> print "\N{U+03B1}"
Wide character in print at (eval 10)[/usr/opt/perl5/lib/5.20.1/perl5db
+.pl:732] line 2.
 [...]
&#945;

  DB<4> print "\N{greek:alpha}"  
Use of uninitialized value $txt in pattern match (m//) at /usr/opt/per
+l5/lib/5.20.1/_charnames.pm line 459.
 [...]
Unknown charname 'greek:alpha' at (eval 11)[/usr/opt/perl5/lib/5.20.1/
+perl5db.pl:732] line 2, within string
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Ubuntu 14.04

 DB<1> use charnames qw(greek)

 DB<2> print "\N{alpha}"
 Unknown charname 'alpha' at (eval 8)[/usr/share/perl/5.18/perl5db.pl:
+732] line 2, within string

 DB<3> print "\N{U+03B1}"
 Wide character in print at (eval 9)[/usr/share/perl/5.18/perl5db.pl:7
+32] line 2.
 &#945;
  
 DB<4> print "\N{greek:alpha}"
 Wide character in print at (eval 10)[/usr/share/perl/5.18/perl5db.pl:
+732] line 2.
 &#945;
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Guess there is something messed up with the AIX character-table?