perlPackages.FileSlurp: silence perl 5.24.1 warnings

this silence annoying warnings happens in nixos-rebuild

activating the configuration...
setting up /etc...
syswrite() is deprecated on :utf8 handles at /nix/store/d51apm4c8mr5hiwh134vdkzgivf4hrl7-perl-File-Slurp-9999.19/lib/perl5/site_perl/5.24.1/File/Slurp.pm line 506.
syswrite() is deprecated on :utf8 handles at /nix/store/d51apm4c8mr5hiwh134vdkzgivf4hrl7-perl-File-Slurp-9999.19/lib/perl5/site_perl/5.24.1/File/Slurp.pm line 506.
...
syswrite() is deprecated on :utf8 handles at /nix/store/d51apm4c8mr5hiwh134vdkzgivf4hrl7-perl-File-Slurp-9999.19/lib/perl5/site_perl/5.24.1/File/Slurp.pm line 506.
setting up tmpfiles