It's such a common idiom that the -p and -i command-line flags were invented to deal with it:

#!/usr/bin/perl -wpi
s/string1/string2/g;
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And then you'd run it as: myScript *.htm

update: note that the contents of a script run with -p or -n are inside a loop. If you want to initialize anything before the loop, use a BEGIN or INIT block.