If 0, 1, 2, 100 in the examples above were used more than once in your program, would you create a symbolic name for them?

Not for the uses in your examples.

I do use enums for 0 and 1 where they are indexes in multi-dimensional arrays with the 0th element being an X coordinate and 1 the Y coordinate; and similar examples.

This is the results of a grep for "enum" in *.pl files in my junk directory:

1031775.pl:use enum qw[ NAME FREQ LEFT RIGHT ];
1161363.pl:use enum qw[ IN DBI ];
1161647.pl:use enum qw[ IN DBI_ENUM ];
1177385.pl:use enum qw[ IN DBI_ENUM ];
904729-2.pl:use enum qw[ CODE GRAPH START PATH SEEN ];
904729-3.pl:    use enum qw[ CODE GRAPH START PATH SEEN ];
CIDR.pl:use enum qw[ CIDR NETWORK SIZE RANGE ];
CIDR.pl:use enum qw[ FIRST LAST ];
dumpTree.pl:use enum qw[ LEFT RIGHT ];
dumpTree.pl:use enum qw[ WIDTH DEPTH NAME ];
FPstuff.pl:use enum qw[ X Y ];
GraphBench.pl:    use enum qw[ CODE GRAPH START END PATH ];
GraphBench.pl:    use enum qw[ CODE GRAPH START END PATH ];
hFP.pl:    use enum 'KEY', 'VAL';
lazyTree.pl:use enum qw[ LEFT RIGHT ];
philo3.pl:use enum qw[LEFT RIGHT ALL];
plotTSP.pl:use enum qw[ X Y ];
plotTSP2.pl:use enum qw[ X Y ];
plotTSP3.pl:use enum qw[ X Y ];
SameFringe.pl:use enum qw[ LEFT RIGHT ];
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