As somebody heavily invested in Perl (just check my nickname, and I've written books about Perl (though Perl 6, not Perl 5)), my impression is that the size of the Perl community is rather stable.

In an overall growing market of programmers and projects, this means a decline of relative market share.

Sources:

* Every four years I organize a German Perl Workshop, and the attendance is roughly the same each year (80-120 people)

* Take a look at the commit stats of the Perl 5 interpreter at github, looks pretty stable over the past years: https://github.com/Perl/perl5/graphs/contributors

* Just general, unquantified impressions from Perl communities I vaguely follow (twitter, Perl questions on Stack Overflow, some IRC channels etc).

* Work use of Perl and and hiring for Perl positions